Prenatal development is the process by which a baby develops inside the mother’s womb. The development takes about 38 weeks to complete. During this time, a single cell becomes a full-term baby.This time period is divided into different categories of development. The 3 primary stages of prenatal development are the germinal period, the embryonic period, and the fetal period. Each of these stages is important for reaching the ultimate goal of a healthy baby.0 to 04th week of pregnancyStage 1 The Germinal stageA period of development from conception (fertilization) to implantation takes about 2 weeks, the 2nd – 4th weeks of pregnancy.
The first 2 weeks of prenatal development are called the pre-embryonic stage.Conception, also known as fertilization, occurs when a sperm cell combines with an egg cell to form a Zygote.At fertilization, the sex of the fetus is already determined, depending on whether the egg receives an X or Y chromosome from a sperm cell. If the egg receives an X chromosome, the baby will be a girl; a Y chromosome means the baby will be a boy.
The zygote moves along the Fallopian tube toward the uterus.This stage is characterized by a rapid cell growth (doubling twice daily) and cell division and the process is called mitosis.Once it reaches the uterus, the zygote becomes what is called a blastocyst, and it begins to implant into the wall of the uterus. It has also begun to produce the pregnancy hormone HCG, which tells the ovaries to stop releasing eggs.This process is called implantation and takes about a week to complete (about 3rd week).
One key feature of the germinal stage is the formation of a tissue called the Placenta. The placenta has two important functions:Passing oxygen and nutrients from the mother’s blood into the embryo or fetusRemoving waste materials from the embryo or fetusThe germinal period ends when the blastocyst is fully implanted into uterine tissue, at about 10 days of gestation.04th to 08th week of pregnancyStage 2: The Embryonic StageThis stage lasts from implantation until about 8 weeks from the time of conception (or the 10th week of pregnancy). This is the most important time of prenatal development as it is the start of development and training of the most important organ systems.The developing ball of cells is now called an Embryo. In this stage, all the major organs form, and the embryo becomes very fragile. Through a process called differentiation, cells begin to take on different functions, like the division between the cells that will make up the supply, also called the placenta (including umbilical cord) and the cells that will develop into the actual embryo.The placenta begins to develop and becomes anchored in the lining of the uterus. In exchange with the maternal blood, the placenta later takes over the supply of oxygen and nutrients to the unborn child. Other cells produce the amniotic sac and the amniotic fluid.Also the process of gastrulation starts by forming the germ layers.The outer layer is called the ectoderm, the middle layer is called the mesoderm, and the inner layer is called the endoderm.Each germ layer will differentiate into different structures.The ectoderm will form many outer tissues such as skin and hair, as well as most of the nervous system tissues – including the brain. The mesoderm will form tissues inside the body such as the lungs, bones, and muscles – including the heart. The endoderm will form tissues such as the digestive tract and bladder, as well as other internal organs.By the 5th week, the embryo resembles a tadpole more than a human, but it is growing fast and is the size of an apple seed. The circulatory system is beginning to form, development of the brain and spiral cord are under way. The tiny heart starts to beat and facial features begin to form.By the 7th and 8th week of pregnancy most of the organs are laid out now, in part they are already starting to work and to coordinate with each other. The face is formed: mouth, nose and nostrils are already distinguishable, the eye sockets are still located laterally near the temples. Eyelids develop. The ear system and the outer ear arise. The brain is also developing rapidly, nerve cells have started sending signals and responding to more and more impulses. The genital organs grow and become sex characteristics. Women have two X chromosomes, and men have an X and Y chromosome; each sperm carries one of the two, leading the male’s sperm to determine the baby’s sex. Humans have about 25,000 genes, but only one gene located on the Y chromosome is required for male development.The proportions of the embryo are still unbalanced – it dominates the head, the upper body is considerably larger than the lower half of the body. The hands are more developed than the feet. Arms and legs only gradually form the right proportions. The body grows from top to bottom and begins to straighten up. The spine develops.All of the essential structures have been formed (both inside and outside) by the time the embryonic period comes to an end and by then the embryo is only about the size of a grape, around 2 cm.08th to 12th week of pregnancyStage 3: The Fetal StageThe last stage of prenatal development is the Fetal Stage, which officially begins after the ninth week of pregnancy.The fetus has completed the most critical portion of development. His skin is still translucent, but his tiny limbs can bend and fine details like nails are starting to form.The entire organ system is now developed and the fetus is almost fully formed. The yolk sac, which was responsible for the supply and metabolism of the embryo in the first few weeks, is no longer needed and gradually decreases in size. Its tasks are taken over by the placenta and the fetus’ organs.In this rapid development period of the fetus, the risk of a maldevelopment and thus a miscarriage is increased. If the infantile attachments exclude a healthy progression or even viability, it usually results in an early miscarriage.By now there is a high probability to diagnose via ultrasound a multiple birth.Arms and legs are already well trained. The fingers on the hands are clearly distinguishable and the toes begin to form on the feet. Soon, targeted movements of individual limbs can be seen but not yet felt from the expectant mother.By the 11th and 12th week of pregnancy the lungs are already developed, but they will provide the child with oxygen only after his birth. Until then, the pregnant woman provides life-saving oxygen in exchange for her own blood. There are three bloodstreams in the umbilical cord. Fresh, oxygen-rich blood flows through the thicker vein into the bloodstream of the fetus and the two thinner arteries are used to transport the waste.The placenta carries oxygen and nutrients from the pregnant women’s blood to the unborn child, filtering out many but not all of the contaminants. The fine vascular system of the placenta transfers the important substances to the blood of the fetus. The bigger the fetus gets, the higher is his need for adequate supply.Therefore, the placenta grows to the same extent as the unborn child. At the end of the twelfth week of pregnancy, the placenta is about the size of a fist.In the last week of the first trimester, the fetus’ tiny fingers now have fingerprints and the veins and organs are clearly visible through the skin; the body is starting to catch up with the head, which makes up just a third of the body size now.After the inner sexual organs, the external sexual organs have now developed. In the testicular, the male hormone testosterone is formed and the female ovaries produce oestrogen. At the end of the third month of pregnancy, the fetus is about 5.5 cm, the size of a hen’s egg.12th to 16th week of pregnancyThe amniotic sac, in which the fetus floats, consists of two skins. It contains the amniotic fluid, a salty and pleasantly warm fluid, in which it is well protected. As the unborn child grows, the amniotic fluid also increases and the amniotic sac enlarges.From the 11th week, the skin is largely developed and well adapted to life in the water. It is protected by the ћvernix caseosa”, a kind of waxy protective coating (waterproof) that covers the whole body. The result is first hairlines, the so-called “Lanugo” – a light, soft hair fluff that covers the whole body.More and more, the movements of the unborn appear purposeful. Gradually it can bend and stretch arms and legs, its hands are even looking for the mouth. Occasionally, swallowing movements can be observed on an ultrasound but its movements are still not noticeable to the pregnant woman.The kidneys are producing urine, which the fetus releases into the amniotic fluid. This process will keep up until birth.In the 14th week of gestation the liver starts making bile, a sign that it is doing its job right and the spleen starts helping produce red blood cells.The fetus is active around the clock, interrupted only by short sleep phases. It still has no day-night rhythm. Only in the last trimester of pregnancy it will have several hours of sleep.Eyelids have formed over the eyes. They are still closed and will not open until the 26th week of pregnancy.The growing body needs more stability now. The still very soft cartilages gradually become harder and it forms first bone tissue. The fingernails become visible and toenails start growing.The legs are much more developed, and the head is more erect than it has been. The patterning of his scalp has begun though the locks are not yet recognizable.The fetus’ heart is now pumping about 25 quarts of blood each day and this amount will continue to increase as the fetus develops.The uterus, amniotic sac and placenta continue to grow. The uterus has now reached about the size of a handball.Around the 15th week of gestation the fetus gender can be diagnosed on the ultrasound.In ultrasound examination, the size of the fetus can now be measured from head to rump (CRL). At the end of the 16th week of pregnancy, it measures about 12 cm from the head to the rump and weighs about 100 grams, about the size of an avocado.16th to 20th week of gestationThe fifth month of pregnancy offers a special surprise; the first child movements are felt by the expectant mother.In these weeks, most pregnant women feel the first child movements – as a gentle kick, as a light knock, flutter or twitching. For many expectant mothers this is a very special moment.Ears, nose, mouth and eyes are in the right place, the unborn child has a clearly infantile face. First hairs are sprouting on the scalp.The fetus can move the joints and the brain is designating specialized areas for smell, taste, hearing, vision and touch.A protective covering of myelin is beginning to form around the nerves, a process that will continue for a year after the child is born.Arms and legs are now in proportion to each other and the rest of the body. The fetus’ kidneys continue to produce urine; the fetus itself does its usual exercise training, like moving around, curling up, sucking its thumb or swallowing, which is a good practice for its digestive system. The fetus will start producing meconium, a black, sticky by-product of digestion, which will accumulate in its bowel.In terms of developmental history, it is crucial for newborn mammals to be able to reach the source of food as soon as possible after birth and to be able to drink from it. For human children, sucking and grasping is essential and the fetus is practicing this now again and again. When the hand reaches the mouth, the fetus sucks reflexively on its thumb. Sometimes the fingers also surround the umbilical cord.At the 18th weeks, the fetus is about 14 cm long (roughly 5 1/2 inches from head to rump) and weighs about 200 grams (almost 7 ounces).At week 20, the fetus measures about 16.5 cm (6 1/2 inches) from head to rump or 25.5 cm (10 inches) from head to heel, and weighs roughly 300 grams (10 1/2 ounces), about the length of a large banana.Before the 20th week the fetus’ measurement is taken from crown to rump, also called CR(L), because the legs are curled up against its torso.Starting from the 20th week of gestation the measurement will usually be in CH(L), which means crown to heel measurement.