Prenatal development and the fetal period
Prenatal development is the process of growth and development inside the womb. Prenatal development is when an embryo or fetus gestates during pregnancy which lasts for about 38 weeks. The period in which this growth and development take place is called the prenatal period. Development is quick during the prenatal period. This period is generally divided into three further stages or periods – the germinal stage, the embryonic stage, and the fetal stage.
The germinal stage
In the germinal stage, conception occurs when a single-celled zygote, i.e., a cell formed by the combination of a sperm and an egg, is formed. After 36 hours after conception, the zygote starts to multiply quickly. During the first two weeks, the cells multiply in numbers. The next three weeks, these separate into two distinctive masses. The outer cells will eventually become the placenta and the inner cells of the embryo.
This cell division continues for almost a week. The cells develop into a blastocyst. This blastocyst is made up of three layers which later develop into different structures of the body. The three layers are namely the ectoderm (which develops into the skin and nervous system), the endoderm ( which develops into the digestive and respiratory system), and the mesoderm (which develops into muscles and skeletal system).
The blastocyst moves along the fallopian tube and attaches itself inside the uterus. This process is called implantation. Implantation takes place when the cells attach themselves to the uterine lining and rupture tiny blood vessels. The connective web of blood vessels and membranes formed between them will provide nutrition for the development for the next nine months.
This implantation process takes about a week, and if the implantation fails, the pregnancy terminates. Studies show that almost 60% of all natural conceptions are never successful. If the implantation is successful, the hormonal changes stop your menstrual cycle, and you begin to experience many physical changes.
The embryonic stage
The start of week three marks the beginning embryonic period. In this stage, the ball of cells is called a fetus which starts to form all major organs, and the embryo becomes distinct as a human and becomes very fragile. The embryonic stage plays a vital role in the development of the brain. After four weeks of conception, the neural tubes develop and later develop into the central nervous system, including the spine and brain.
Around this time, the head begins to form, followed by eyes, nose, ears, and mouth. The blood vessels become the heart, and during the fifth week, the buds which will form legs and arms appear. By the end of the eighth week, all major organs are developed except the sex organs. At the end of this period, the embryo weighs about one gram and is about one inch long.
During this stage, the most considerable risk to the embryos is the teratogens which are agents such as viruses, drugs, or radiation that can cause deformities in an embryo or fetus. Teratogens are any agents which cause embryonic malformation, i.e., congenital disabilities. Cigarettes, alcohol, cocaine, warfarin, ACE inhibitors, and Accutane are examples of some teratogens that affect fetal development. At the end of this period, the embryo is about one inch long.
The last stage of prenatal development is the fetal stage or “fetal period.”
This last stage of prenatal development occurs from the ninth week after conception and lasts birth. In this period, the fetus approximately gains 7 pounds, and the organs continue to develop. About one month into this stage, the genitals of the fetus begin to develop. During the last three months, brain development increases rapidly in size, an insulating layer of fat forms under the skin and the respiratory and digestive systems start to work individually.
The head makes up nearly half of the fetus size, and the four heart chambers finish development. The fetus quickly develops bones and muscles, and it begins to move inside the uterus. This last stage is the longest stage in which pregnant women get an ultrasound done.
The following developments that take place in the fetal period are –
Into Week 10: Fingernails and hair start to grow. The heart, hands, feet, brain, and other organs are present but are still in the developing stage.
Into Week 11: Nearly all structures and organs are formed by now. Fingers and toes are separated, and the genitals begin to take the form of proper gender characteristics.
Into Week 12: The digestive system and liver function independently, and the pancreas makes insulin.
Into Week 13: The fetus begins to get its essential nutrients from the placenta, and the veins and organs are visible through the skin.
Into Week 14: The kidneys start to produce urine, and the liver begins to make bile. In boys, the prostate gland develops, and in girls, the ovaries move from the abdomen to the pelvis.
Into Weeks 15 to 16: The heart pumps out 25 quarts of blood a day, and the fetal structures look more normal.
Into Week 17: The fetus starts to move its joints, and the retina becomes sensitive to light.
Into Week 18: The fetus starts to hear and gets startled by noises. The fetus’s skin starts to grow a wax-like protective layer and tiny air spaces begin to form in the lungs and the vocal cords.
Into Week 19: The brain is assigning areas for the five senses.
Into Week 20: The fetus weighs about 10 ½ oz, swallows more, and produces meconium.
Into Week 21: The eyebrows and lids develop, and for the female fetus, the vagina begins to form.
Into Week 22: Tiny tooth buds beneath the gums develop, and the eyes are formed, but the irises lack pigment.
Into Week 23: The fetus weighs over a pound, and it can feel movements and hear sounds. Blood vessels in the lungs are developed, enough to prepare for breathing.
Into Week 24: The fetus is almost a foot long. The lungs are developing branches of the respiratory tree and cells that produce surfactant, a substance that will help the air sacs inflate once born. The lungs are not yet mature.
Into Week 25: The amount of body fat rapidly increases. The bones are completely developed but are still soft and pliable. Thalamic brain connections form that mediates sensory input. Iron, calcium, and phosphorus become more abundant. Tiny breast buds are present in both sexes.
Into weeks 26 -28: The fetus is 15 inches in length and weighs approx. 1.3 kgs. This week, the fetus has rapid brain and nerve development. It starts to gain control over the movements such as opening and closing eyelids, etc., by this time; the lungs have developed to the extent where breathing air is possible.
Into Weeks 29–32: Fat deposits become more prominent under the skin. The lungs remain immature, but breathing movements begin, and the fetus’s bones are developed but not yet hardened.
Into Weeks 33–36: The fetus is now approximately 16–19 inches in length and weighs 2.6–3.0 kg. Body fat continues to increase, and fingernails are fully grown, and the fetus has gained a high degree of control over body functions.
Weeks 36–38: The fetus reaches 19–21 inches in length and is considered full-term by the end of this period. Lanugo has disappeared chiefly and is replaced with thicker hair on the head. Fingernails have grown up to the tips of the fingers.
Birth is approaching and occurs around the 40th week. The fetus is considered full-term after weeks 37, and before 40, it means the fetus is sufficiently developed to sustain the world outside the uterus. Although the fetus is ready for the world outside the brain, development does not stop after birth.
A considerable amount of brain development occurs postnatally, including growing in size and volume while changing in structure. The size of the brain increases between birth and preschool. As children learn and have new experiences, some brain networks are strengthened while other connections are lost.