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Gestation Periods - 07/11/2008

Gestation Periods - Humans & Mammals

Condoms help prevent gestation. Their major role is to block the union of the sperm and female egg so that no fertilization takes place thereby preventing a woman from getting pregnant. For this reason, condoms are considered very effective in preventing pregnancy and the spread of sexually transmitted diseases notably in humans. Many users can attest to this and various non-profit organizations worldwide have actually been urging people, from teenagers to sexually active adults, to use condoms for the same reasons. Countries with large populations have also turned to this birth control method to minimize the increase in birth rates.

While humans can have as many children as they want, it is not ideal for women in poverty-stricken areas to do this. One reason is that poor families are not capable of feeding their children and giving them the proper education as they grow. And in countries that have huge population, there’s a tendency that a nation’s wealth will not be equally shared. As a result, various non-profit organizations around the world have been strongly advocating birth control programs including the use of condoms and other contraceptives to curb the increase in birth rate.

Mammals, on the other hand, differ from humans as they can freely produce as many offspring as they want. They need to do this to save their species from extinction and their continued existence is highly important in ecology. Condoms are not their thing. They don’t have the intelligence to use condoms like humans.

Humans and animals also differ in their gestation period. While humans have a fixed length of pregnancy, mammals in particular vary in the length of time they carry their babies in their womb. Another major difference that’s noteworthy is in the number of babies they produce. While most pregnant women give birth to only one baby, medium-sized mammals give birth to a litter or multiple offspring. Only the large mammals normally deliver a single baby.

Gestation in humans

Gestation simply refers to the carrying of a fetus in a woman’s womb or uterus. This is also referred to as pregnancy which normally takes nine months or specifically from 37 to 42 weeks. The average length of gestation, though, is estimated at 40 weeks. The reason for this is that pregnancy begins from the first day of a woman’s last menstrual period and not from the date of conception. In truth, the date of conception takes place two weeks after pregnancy begins.

Sometimes, though, there are women who failed to keep track of their last menstruation notably those with irregular periods. What medical experts do then is consider a pregnancy in its full term if the expected due date falls between 37 and 42 weeks. In medical terminology, the due date is called estimated date of confinement (EDC). Not all pregnant women, though, deliver their baby on the exact EDC given.

There are various ways to calculate the due date. With the advent of the internet, this has become very convenient. There are now many online pregnancy calculators on the web.

The simplest way to determine the due date is to just add seven days to the date of the first day of your last menstrual period and then add nine months. So, for instance, if the first day of your last period is March 10, add seven days (that’s March 17) and add nine months which results in December 17.

But then, not all women have regular periods and those with irregular menstruation often find it hard to determine the date they started conception. And so, there are other ways that obstetrician-gynecologists (OB-Gyn), or doctors specializing in childbirth and the treatment of disorders in women’s reproductive system, use that enable them to estimate the length of gestation. One way is through an ultrasound examination done between six and 12 weeks. Other ways include determining the size of the uterus during vaginal or abdominal examination and the time when the first fetal movements were detected.

A pregnancy ultrasound is a non-invasive test that uses high frequency sound waves to scan the fetus and the mother’s reproductive organs. It is used to estimate the baby’s due date as well as diagnose other conditions such as the presence of multiple fetuses, health problems of the baby, ectopic pregnancy, abnormalities of the placenta and the reproductive organs of the mother.

The three trimesters

The gestation period starts when a female egg is fertilized by a sperm inside the fallopian tubes. This important period in humans is divided into three trimesters with each trimester taking three months for a total of nine months of pregnancy.

The first three months are critical and involve a lot of adjustment on the part of the mother. Some first time mothers have mixed emotions from fear to anxiety to hope for the baby’s future. Sometimes, the father also experiences unusual feelings as if he’s conceiving himself. Sexual activity among couples may even be reduced for fear it could affect the baby inside the womb. Physical changes start to show during this stage sometimes causing much discomfort. These include a feeling of bloated breasts, nausea and vomiting causing loss of appetite at times, frequent urination, fatigue, weight gain and increased basal temperature.

First month – embryo implants on the uterine wall, heartbeat starts on the 25th day, arm buds show on the 26th day, leg buds show on the 28th day, embryo is 1/5th inch long.

Second month – the face forms, ears, spine, brain, liver, kidneys, digestive system start developing, arms and legs developed, fetus measures 1 and 1/18th inches.

Third month – facial features present, nose and outer ears formed, head movement and sucking starts, teeth buds form, hair appears on head, all internal organs formed, swallows amniotic fluid, fetus about three inches long and weighs one ounce, placenta fully developed.

Fourth month – fetus is complete with strong heartbeat, can move, kick, sleep and swallow, skin is pale and transparent, measures 6-7 inches and weighs about five ounces.

Fifth month – fetus grows and moves from side to side or somersaults, may suck its thumb, is about 8-12 inches and weighs ½ to 1 pound.

Sixth month – fetus is covered with soft hair known as lanugo, skin is thin, wrinkled and red, eyes start to open, 11 to 14 inches in length and weighs 1-1 ½ pounds.

Seventh month – fetus grows and gains weight rapidly, kicks and stretches, sucks its thumb, opens and closes eyes, 15 inches long and about three pounds.

Eighth month – less fetal movement due to big size, stronger kicks, head bones still soft and flexible, about 18 inches long and five pounds.

Ninth month – fetus gains ½ pound every week, has mature lungs, settles in good position for birth with its head down and knees curled up high against the chest.

Phases of human development

There are four phases in the early development of humans. These are the fertilization stage, implantation, gastrulation and embryogenesis.

The first stage is fertilization which refers to the union of a sperm and the female’s egg or ova. This is followed by the egg’s second division that takes place in the fallopian tube or the oviduct. After this, the egg is now scientifically called a zygote and continues to travel down the fallopian tube to the uterus.

The second stage is known as implantation. This is when the fertilized egg gets implanted on the uterine wall and cells are divided and organized into a two-layered disc. After implantation, the placenta, a rich mass of tissue filled with blood vessels, is formed. While this takes place, another group of cells separates from the developing embryo and develop into the membrane that will surround the fetus or the embryonic sac. This sac is filled with fluid and helps protect the fetus as it develops into the later stages. This phase starts during the second week of development.

Gastrulation is the third phase and occurs during the third week of fetus development. During this time, the embryo forms a three-layered disc. Three germ layers are then established – the endoderm, ectoderm and the central mesoderm. These three later develop into the different organ systems. It is the mesoderm that forms the umbilical cord which is crucial in delivering nutrients and wastes between the fetus and the placenta.

Finally, the last stage is called embryogenesis. This refers to the real embryonic phase which takes place from the third week through the eighth week after conception. The organs are developing during this phase. At the end of this stage, the circulatory system which involves the heart or the largest organ is already functional. The nervous system is still in a stag of cell division.

Fetal period

After the embryogenesis, the last stage of human development is called fetal period taking place from nine weeks to the 38th week. It is during stage when the fetus forms its appearance.

Two processes are involved here. The first is known as rapid growth or an increase in the fetus’ size and cell number. The second process is the differentiation of tissues and organs to be able to perform their distinct functions.

During rapid growth, the growth rate of the fetus is at its peak and this occurs until the 16th week. At this time, the weight of the fetus increases 25 times. However, the biggest rise in weight gain happens during the last month of gestation. During the last four weeks, the baby gains as much as 500 grams which is the average weight it gains during the first 20 weeks of development.

Full term babies normally weigh an average of 7.7 pounds or 3,500 grams. Infants weighing around 2.2 pounds are called immature while those weighing about 5.5 pounds are premature. Those weighing less than 500 grams or less do not often survive. Weight is just one way of gauging the age of a fetus. Length is another way to estimate a baby’s age and its chances for survival.

Fetal growth is influenced by various factors. These include the mother’s nutrition, social habits, condition of the placenta and the genetic makeup of the fetus.

There’s actually a medical sub-specialization known as perinatology which deals with the mother and fetus during the third trimester or last three months of pregnancy to about one month after birth. A perinatologist analyzes the fetus in a variety of ways. One technique is called amniocentesis which enables the examination of amniotic fluid taken from the mother’s amniotic sac for metabolites and fetal cells.

As for the fetus, it can be examined via the ultrasound and fetoscopy. Ultra sonography is non-invasive and makes use of ultrasonic waves. Fetoscopy is an invasive method which involves the insertion of a needle into the amnion. This technique is used not only the find out the fetus’ condition but even to take a sample of fetal issues for biopsy.

The second process involves the development of tissues and organs. During the earliest stage of fetal development notably in the first trimester, the head is the most dominant part of the body that can be seen and accounts for almost half of the fetus’ length. The face is still flat with eyes wide apart and ears low. At this time, the intestines protrude temporarily through the abdominal wall and the external genitalia appear the same between the sexes. The fetus also excretes urine into the amniotic cavity.

The rest of the body development occurs after the fourth month when the limbs start to grow. The first movements of the baby inside the womb or quickening can be felt by the fifth month. Fine hair also begins to develop. By the sixth month, the fetus can breathe through its lungs. This, however, depends on the protein-lipid complex called surfactant secreted from lung cells. If the surfactant is insufficient, babies born prematurely may like suffer from respiratory distress syndrome.

The nervous system of a fetus develops reflex responses as it moves into its seventh month. At this stage, the fetus reacts to light by constricting its pupils, it can control its breathing and swallowing. As for the cardiovascular system, it normally experiences major changes during the time of delivery.

As the fetus is nearing its full term, fat tissue is deposited in its body. The stomach’s circumference becomes slightly bigger than the head. What some people find it incredible and hard to comprehend sometimes is how the baby passes through the birth canal which may not be as big as the baby. The reason is that the fetus’ skull still has flat bones separated by the connective tissue known as fontanelles. This enables compression of the head during delivery or the process called molding. There’s actually no need to worry when the head appears misshapen after birth as it will quickly go back to its normal shape.

The process of giving birth is also referred to as labor and has three stages. The dilation of the cervix is the first stage, then the actual delivery of the baby and finally, the expulsion of the placenta.

Premature and overdue

If a woman gives birth before reaching her 37th week, her baby is called premature or preterm. Most often, a baby’s chances to survive is higher if it is closer to the estimated date of confinement or due date. Most babies born after 34 weeks of gestation and given great pediatric care will most likely survive.

Preterm babies including those with low birth weights account for the second leading cause of infant death or 17 percent. These premature babies are prone to health problems because of their immature internal organs. The most common problems they encounter include difficulty in the respiratory system and higher susceptibility to infection.

On the other hand, a baby that has not been born way past the due date is called overdue. When this happens notably after two weeks past its expected date, labor is usually induced. Pregnant women who are one week past their due date without showing any signs of labor are closely monitored by their doctor. More tests may be conducted such as the monitoring of the baby’s heart rate using a cardiotocograph machine and ultrasound scans.

The placenta usually begins to deteriorate after an average of 38 weeks of gestation. In this case, the baby inside may not have sufficient oxygen supply. Another possibility is that the fetus could grow too big for vaginal delivery. When this happens, a caesarian section delivery is the most likely option.

Important study

A new research by the Hamilton Health Sciences has found that the brains of babies born very prematurely do not develop well compared to full term babies born. Specifically, an ultrasound examination of the brains of babies born around only 26 weeks of gestation showed that certain aspects of brain development were affected.

Dr. Sandra Witelson, a professor of psychiatry and behavioral neurosciences at the Michael G. DeGroote School of Medicine at McMaster University and chief investigator said in very premature babies, a part of the brain does not grow in a normal way after birth. Their study also found that some parts of the brain did not show any changes from the day the babies were delivered until they reached their supposedly full term birth date.

The study involved 80 premature boys and girls weighing less than 2.2 pounds and born 26 weeks. The babies’ brains underwent ultrasounds at birth and before they were discharged from the hospital at already around 36 weeks. Their ultrasound results were then compared with those of 38 full term babies also at about 26 weeks gestation and at birth.

The study’s results are considered relevant in how premature babies are cared for. They point out that the early brain may be affected when subjected to complex stimulation at a very early stage.

Gestation in mammals

Approximately 4,500 species of mammals exists in the world. Mammals are said to be the best-developed vertebrates and posses highly developed nervous systems compared to other animals. The largest is the blue whale with a length of 30 meters and a weight of up to 190 tons. The African elephant is the biggest mammal living on dry land weighing up to 5.7 tons.

The common characteristics that mammals share with humans include the ability to nourish their young with milk from their mammary glands, the presence of hair as body covering and the existence of a muscular diaphragm. All mammals produce live offspring that go through the early stages of development in the womb of the mother. At birth, some mammals are helpless while the others are able to walk and run right away with eyes and ears open.

Mammals are bisexual and fertilization takes place internally. This only means that embryo develops inside the mother’s body similar to humans. How long the gestation takes place, however, varies among mammals. Smaller animals like rodents have shorter gestation periods and they can reproduce as often as six to eight times each year. In bigger animals, gestation takes longer and they can deliver their young once in several years. The number of their offspring also differs usually depending on the size. Smaller animals tend to have a lot of newborns while big animals often have only one.

Mammals conceive immediately after mating. The period of gestation may take as short as 12 days to more than 20 months in the case of the African elephant. In a few species such as bats, rodents and kangaroos, embryo development can temporarily be stopped at a very early stage. This is known as delayed implantation or embryonic diapause which enables the adjustment of the time of birth. In rodents, this system helps to ensure the proper spacing of successive litters. In kangaroos, it enables the birth of a young joey to be put on hold until rain produces enough food for them.

In mammals, gestation begins when a fertilized zygote implants in the uterus of a female. Did you know that mammals can actually have one or more gestations at the same time during their pregnancy?

The shortest gestation period is estimated at only 12 days. The mammals that fall under this category are the American or Virginian opossum, the rare water opossum and Australia’s eastern native cat. These three animals give birth to their babies even while still at the embryonic stage. The young then develop into maturity for about two months inside a small pouch similar to a kangaroo. From there, they continue to stay close to their mothers normally holding tight on the back for another month.

If the shortest takes only a few days, the longest gestation in mammals takes almost two years. The record holder is the African elephant which carries its baby for approximately 660 days or about 22 months. Elephants are the biggest land animals that still exist today. Only three species of elephants are living – the African Bush elephant, African Forest and the Asian elephant. A newly born elephant would normally weight 120 kilograms while its lifespan may reach 70 years or even longer. Also, the infants are weaned only after their first year of life.

Next in line for having long gestation periods are killer whales, 517 days, followed by the giraffes and camels which carry their babies in their wombs for 400 days. Baby killer whales or commonly known as the orca, the largest member of the dolphin family, are born in the autumn although mating takes place all year round. Female orcas give birth to only one baby killer whale at a time. After birth, the baby whale stays close beside its mother for another year before it is weaned.

Mammals with gestation periods almost similar to humans are the dolphins (276 days), chimpanzees (237 days) and cattle (280 days).

Other mammals and their gestation periods are bats 5-34 depending on species, hamster – 16, rat – 21, bear – 31, rabbits – 33, kangaroo – 40, fox – 42, squirrel – 44, cats and skunk – 62, dogs – 65, guinea pig – 68, lions – 108, pigs – 115, sheep – 150, goats – 165, baoon – 187, black bear – 210, grizzly bear – 225, polar bear – 240, reindeer – 215-245, cows – 280, horses – 336, giraffe – 395-425, camel - 406.

In dogs, the period of gestation may vary depending on whether the bitch has been bred two or three times or whether the eggs are fertilized after a day or two days following the mating stage. Eggs can also remain fertile for about two days and the sperm can stay in the vaginal tract for several days.

Small sized cats carry their babies for about two months while the bigger ones for about four months. Each litter consists of one to six kittens. Breeding happens in the later part of winter or early part of spring. However, some members of the cat family such as the lions, tigers and leopards can breed during any part of the year.

Canines give birth to about four to six young after 51 to 80 days of gestation. Among the carnivores, the Arctic fox produces the biggest litter, about 11 on average but other times, they can be 20 or more.

As for orangutans, they usually carry their young in the womb for about eight months. They give birth once every eight years. Wild females first give birth when they reach the age of 15 or 16. Those in captivity can, however, give birth at a young age of seven.

Where bats are concerned, everything in their sexual cycle is synchronized – from the gestation, birth, lactation and weaning. It’s only in the gestation period where they differ depending on the species.

Black bears mate in June and July but implantation of the fertilized egg is delayed and takes place in November or December. Gestation takes from 60 to 70 days and by January or February, four cubs are born. The cubs come out as blind, with hair and no tooth and they stay with their mother for 16 months.

The brown bears deliver their babies during the winter season. The cubs are usually twins and were developed in their mother’s womb for about six to eight months. A cub weighs less than one kilogram at birth.

Camels have a gestation period of about 14 months and they give birth to their young during the rainy season.

Wolves breed between the months of February and April and give birth to about six puppies in the spring. Gestation lasts for about two months. Their favorite place of delivery is the den notably a natural hole in a hillside. Otherwise, a hollow log or abandoned beaver lodge will do.

Reproduction

Not all mammals breed all year round. Most of them, though, have specific breeding seasons that allow their date of delivery to coincide with times when food is abundant. In some species, females show signs when they are ready to mate. In most mammals, however, males know when females are fertile by sensing the pheromones or the airborne chemicals. There are some mammals such as cats and horses that can sense these pheromones with their Jacobson’s organ found on the roof of their mouth.

All mammals reproduce through sexual contact which involves two types. In the egg-laying mammals, their excretory and genital organs open into a common orifice known as the cloaca. The sex cells are normally transferred from the male to the female by bringing the cloacae into apposition.

In other mammals, the male sex cells are transferred during copulation. After fertilization takes place, the offspring develops inside the mother’s body. However, for the monotremes, they produce eggs with large yolks and leathery shells. In many marsupials, gestation occurs only from 10 to 15 days with the young developing in the mother’s pouch after birth. All the other mammals stay close to their mother after birth for some time before they can continue an independent existence.

Milk from the mother’s mammary glands nourishes the newborn mammals with water and other vital nutrients. Mammals have a strong sucking instinct that enables them to feed immediately after birth. In marsupials and placental mammals, milk is released by a set of nipples also known as teats. They feed differently, though. While the young placental mammals normally leave their mother’s nipples between feedings, the newborn marsupials are stuck firmly to their mother’s nipples via their mouth and cannot be moved.

If mammals differ in the way they breed, the length of their gestation and how they feed their young, they also differ in the age they achieve sexual maturity. Rhinoceros, for instance, are considered sexually mature at the age of 20.

Fertilization

Fertilization of the egg or ovum in most mammals occurs high up in the fallopian tubes. It is when the fertilized egg moves down the fallopian tubes when the cell undergoes its initial division. The egg initially turns into a ball of cells known as morula. The next process involves the separation of the egg into an inner cell mass and an outer layer of cells and this is known as blastula. Separately, the inner cell mass is called the zygote while the outer layer of cells is called the chorion.

While the cell growth moves down the fallopian tube, it gets nourished by the stored food in the egg. However, nutrition is in short supply in mammals and the zygote will eventually need an additional source of food. From the fallopian tubes, the fertilized egg then enters the uterus or the uterine cavity.

Once inside the uterus, the egg connects with the endometrium of the uterine wall in different ways depending on the species of mammal. From here, embryonic tissues and a host of maternal material are then developed by the placenta. The placenta can actually take five various forms depending on the degree of contact and the number of tissue layers between the maternal and the embryo’s blood supply.

The layers of tissue that develop on the maternal side are known as the endometrial epithelium, connective tissues and the blood vessels’ endothelium. On the part of the embryo, the tissues involved are the blood vessels’ endothelium, the mesenchymal tissues and the chorion. Experts point to the varying lifestyles of the different species of mammals as a major factor in the differences in placenta forms that take place.

Placental mammals

Most mammals are known to be placental mammals including rabbits, cows, yaks, hippopotamuses, bats and humans. In other words, they are developed inside a placenta in a longer gestation period. The placenta here plays a vital role in allowing a longer gestation period. It is the placenta which enables nutrients from the mother to travel to the embryo. It also enables waste products to leave the embryo so they can be discharged by the mother.

The placenta is made up of several layers of material. It is comprised with blood vessels which serve as a courier of nutrients originating from the mother’s system to the embryo. While the white blood cells and other components of the immune system including blood are maintained within their own areas, the nutrients from sugar and fats to the minerals are passed into the embryo and the waste products discharged. This biological fact enables young mammals to stay longer in their mother’s womb and after they are brought out into the world, are already able to run along with their parents after only a few hours or days.

Size matters

It is worth noting that the gestation period of a mammal sometimes depends on its size. In other words, the bigger the animal, the longer is its gestation period. And that’s why the largest animal on earth, which is the elephant, carries its baby inside its womb for approximately 22 months.

Out in the world

Most mammals give birth to their young on their own. However, there are some species such as the dolphins and elephants that get help from non-breeding females during their birthing process. Placental mammals that gather around the animal giving birth often lick the embryonic membranes off the newly born young and nudge them to the mammary glands of the mother for their very first milk. This the complete opposite showed by female marsupials in that they just leave their young to find their own way into the pouch without any help from the mother itself.

Not all mammals, however, are the same especially when it concerns their newly born young. Did you know that herbivores or the plant-eating mammals are actually more capable of standing and moving on their own just a few minutes after they are born compared to the carnivores or meat-eating mammals?

Another interesting to note is that even mammals belonging to a single order differ greatly. For instance, newly born rats have their eyes closed are naked and helpless after a gestation period of 21 days. On the other hand, a baby guinea pig is born with open eyes, covered hair and is almost capable of feeding itself after a 67 day gestation period. Another example is the rabbit born with eyes closed while its relative, the hare is born with open eyes.

Final note

From all these information we have detailed, one thing that can be deduced is the fact that gestation happens only when the sperm meets the female egg and that only humans can make use of the condom as a way of controlling birth. It also serves a vital function in the proper spacing of children and preventing the spread of infectious diseases that can be transmitted through sexual contact. Condoms now come in different forms, in various scents and flavors and manufactured for use by men and women.

Yes, humans may be part of the mammal family but, unlike animals, they cannot just go on having children frequently. Men have to remember that there is a limit to the capacity of a woman’s reproductive system, notably the uterus where the baby develops. While sure, the more children, the merrier the family, a couple should put a limit to the number of children that they want to raise. Times are hard and education is becoming more expensive these days.

 

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