Just What Happens at Conception and Afterwards?
The Early Prenatal Development of a Person
Day Number 1 -- A person's existence, of course, begins at conception. That occurs when a woman's egg (released into the fallopian tube from the mature ovarian follicle) is impregnated and fertilized by a male sperm. The woman's estrogen levels at this time are high in order to help build the endometrial lining, stimulate hormone secretion, and facilitate the production of cervical mucus.
The ovum/egg is surrounded by a protective confine known as the zona pellucida. Therefore, before fertilization can take place, sperm cells must find an entry through this field in order to move into the genetic core of the ovum, the oocyte. When this occurs, it is the conception of a human being. As soon as fertilization occurs, a number of other things happen as well including an immediate change in protein coating around the egg which prevents the entrance of other sperm.
The term "fertilized egg" is not scientifically appropriate but rather, zygote, a single-cell organism that is a beginning human person with a new and distinctive genetic code reflective of the person's sex, ethnicity, appearance and other inborn traits.
But the zygote stage is very brief for, within minutes from conception, the cell begins to divide and multiply at a furious rate as it begins its free-floating trip down the fallopian tube to the uterus. Some of the cells are directly related to the development of the baby whereas others begin to form the trophoblast (the placenta, umbilical cord, and amniotic sac).
Week Number One -- This is a journey that will take as little as three or four days (or even as much as a week or 10 days) before the rapidly-developing blastocyst thoroughly implants itself into in the endometrium of the uterus. When the blastocyst first establishes contact with the endometrium, there is an exchange of hormones that assists in the implantation process. Clearly, it is very important that a thick, vascular-rich uterine lining is waiting to provide the life-sustaining nutrients necessary for the developing person.
The term "embryo" is now normally used of the developing person and will be so used until the 8th week of pregnancy. Then the term "fetus" or, more usual among the interested parties, "baby" takes center stage.
At the core of the cell mass forming the person's body, three different groups of cells begin an amazing action. The mesoderm (middle layer) will form the muscles, the circulatory system, the skeleton, the kidneys and the reproductive organs. Meanwhile, the endoderm (inner layer) will form the respiratory tract and the gastrointestinal tract while the ectoderm (outer layer) develops into the nervous system, senses and skin. By the end of the first week, the single-cell zygote has been transformed into millions of cells in a body that can already been seen without a microscope.
Week Number 2 -- The embryo, already having a powerful effect on the mother's body, produces hormones which will stop the mother's menstrual cycle.
Week Number 3 --The heart begins to beat.
Week Number 4 -- Along with every other body system, the brain is developing. Growth, ripening and specialization just keep happening at a very fast rate. The eyes and ears begin to form as well as an opening for the mouth. The baby's own heart is pumping blood. The arms and legs start visibly progressing.
The placenta begins functioning as the umbilical cord develops.
A sensitive pregnancy test measuring for hCG may well reveal the certainty of the mother's pregnancy by now.
Week Number 5 -- The baby is about 1/3 of an inch long now but expanding by the day. Everything continues apace. Distinct facial features are now apparent and, beneath the surface skin of the baby, his or her eyes have a retina and lens.
Mother probably isn't feeling the baby move quite yet but that's not because he isn't trying. Indeed, the major muscle system is being steadily developed and the child does move within his protective sac. For instance, that part of the nervous system affecting a person's sense of equilibrium is already in place when the baby is disturbed by mother's movement, he or she notices the shift in spatial orientation and thereby shifts his or her own position to re-stabilize.
The spinal cord's growth and solidification is now obvious.
Week Number 6 -- Things are really happening! All internal and external structures are being developed at the same time. In fact, most will near "completion" (except for growth) by the 8th week. The size of the baby now is anywhere between 4-6 millimeters. He or she is still very tiny but not for long.
The larynx is now starting to evolve, as do the workings of the inner ear. Where the nose will protrude is now clear. The heart is bulging out a bit from the body and is certainly not in finished form. But it's beating to beat the band anyway! Blood circulation is underway. The primordia of the liver, pancreas, lungs, and stomach are evident. Transvaginal ultrasound can pick up 86% of the fetal poles with heart motion, and 100% of the yolk sacs at this point.
Week Number 7 -- The fingers and thumb have appeared but are short and kinda' webbed. Compared to the rest of the body, the head is pretty big and will stay like that for a few more weeks anyhow. He or she is still only less than an inch long now but the process is speeding up even faster. He or she will grow about a millimeter a day for awhile.
The skull is translucent and so the brain is plainly visible. The liver and stomach are growing. Normally during this week the bones will start to ossify too while the beginnings of twenty "milk teeth" are being formed in the baby's jaw. Fascinatingly enough (and in this embryonic world, pretty much everything is fascinating!), the prenatal child will go through three sets of kidneys before being born. This week the second set will form.
And, oh yes, though the baby's gender was determined way back there at the moment of conception itself, the genital tubercle is now present. Even the most eagle-eyed ultrasound nurse couldn't spot it yet but it's just a matter of time before Mom and Dad will know which side of the "names list" they have to concentrate on.
Week Number 8 -- Most of the joints are formed. Bones are ossifying. Elbows make the scene. Ears, ankles and wrists are present. The little stubs that will become toes have appeared. By the end of the week, the baby will probably be 3/4 of an inch to a full inch long. The heart has started beating at about 60 times a minute. The stomach is producing gastric juices. The liver is making blood cells. Brain waves demonstrate that it is beginning to work well.
Week Number 9 -- The fetus (Latin for "little one") now has a strongly beating heart underneath a remarkably developed body. The head turns; the mouth opens exposing the tongue; the eyes have eyelids; the nose is prominent. The baby's fingers and toes are more sharply defined. They even have nails. One of those digits, usually the thumb, will soon become a favorite pacifier. Several body parts and organs are developing in symmetry including, respectively, gonads and ovaries.
The "tail" appearance caused by the formation of the spinal cord is now almost completely gone. The digestive system continues to develop along with the intestines within and the anus without.
Movement within the uterus is now frequent.
Week Number 10 -- Vocal chords are there and ready. The inner ear and auricle are complete too. But the world he or she lives in is still pretty silent...except for his heart, mom's heart, and all the other sounds he can pick up through the amniotic fluid from the outside world. The child can cry and, because the brain and nervous system are well developed, the fetus can most certainly feel discomfort and pain.