From Embryo to Baby.

From Embryo to Baby.

Rachel Johnson

You may think it is an easy process going from being an Egg and Sperm to being a baby. We often take for granted the complex world of reproduction. So Step by step lets walk through what it takes to become a baby.

During 7 days a month a woman is fertile. This can be 7 days before menses, or the 7 days of menses, and even up to a couple days after. That is because ovulation (maturing the egg and then releasing it) can happen at any time, once the woman's hormone levels peak. Several eggs actually begin to mature, but normally only one is released, and the other shrink back to size. The egg bursts off of the ovary, and begins floating towards the Fallopian tube.

Sperm, that is released from the male, travels up through the uterus, which is a long and dangerous journey. The environment of the uterus can be deadly for a sperm. The uterine walls normally secrete harmful acids, and it is only during ovulation that the fluid in the walls aid to nourish sperm.  The sperm have to briefly rest and then continue to swim. It can take them two days to reach the egg, and timing is crucial. Once the egg has left the fallopian tubes it becomes less likely pregnancy can take place.

The egg that has been released from the ovary goes into the Fallopian tubes, where it will spend the next two days, awaiting fertilization. If it is not fertilized it will pass out of the uterus with menstruation. If it is then it begins the process of becoming attached to the uterine walls. Which starts with fertilization, goes to cell division, and then to implantation.

First the sperm have to make it through the uterus to the egg though, and though millions will begin the journey, very few will actually make their way to the egg. This process weeds out weaker sperm, or sperm that are mutated so they can not properly swim. The sperm smell the egg, not in the sense that we smell, but in the sense that there is a chemical trail from the egg that they follow. Once they find the egg the sperm attack the outer shell. An acid located just inside the outer membrane weakens the shell so that one sperm can penetrate the wall. Once the first sperm is in then the wall calcifies, and does not allow any other sperm inside. This makes sure only two sets of genes are used to make the offspring. One set from the mother and the other from the father. Once they meet they will cross over and create a genetic hybrid who will later become the offspring.

The tail to the sperm is lost in this process and only the head enters. Then the shell of the head is shed and the two sets of genes begin the dance(crossing over) to make a zygote. The genes will make copies of them selves (4 pairs of genes)and then cross over one another. They will then split into two cells. So now the egg has gone from a single cell to two(2 pairs of genes in each) cells. Then the genes will separate and make four cells. It is at this point that the cells have the right set of genes to continue. And the blastocyst starts towards becoming a zygote. After there are four cells DNA will also be replicating as cells divide.

The fertilized egg moves down the Fallopian tube to the uterus, where it will embed in the wall for the next five days. After that the zygote becomes visible, attached to the uterine wall by the umbilicus. At every point in growth the body is checking each cell, since all cells are very important right now. If the zygote has a cell that is mutated, and the body knows that this will be a fatal flaw, the zygote will send a signal to all cells to go into apoptosis, or cell death. This will cause the pregnancy to terminate.

If all the cells are correct the zygote will continue development. The first visible part will be the heart. At this point gills and a tail are still visible. The zygote looks like a fish. As it matures the genes for gills will turn off and lungs will form. The genes for a tail will turn off and we will have only the remnants, which is the coccyx or tail bone. The fetus will grow hands and feet, and then the brain will begin to form. The brain will not finish till the second trimester.

During this time hormones produced by the zygote, who becomes a fetus keep the pregnancy going. These hormones secreted by the fetus in the placenta cause the mother to grow larger breasts. It keeps her body from cycling for menstruation, and these are removed from her body in urine.

As the fetus grows the uterus extends to meet the demands of the fluid filled sac and the growing fetus inside. During the fourth month of pregnancy the fetus goes from being the size of a bean to being around two inches long. It is also this period of time where sex is determined by the induction of hormones. This and the X or Y chromosomes determine sex. The induction of testosterone at the right time makes the fetus male. Then he begins to develop testicles. These are attached to a ligament that goes up close to the kidneys. Now normally the testicles are right in the inguinal area. That means down by the lower abdomen. They usually drop into the testicle sac, where they will stay. Under some circumstances the ligament does not grow long enough to allow them to descend and so they are not in their proper place. Usually testicles that are up too far in the body do not function normally as the heat causes problems with proper production of hormones, and sperm.

The female fetus is almost the same, her ovaries are attached to the body on a ligament, and the ovaries are at the top parallel to the uterus, so that eggs once released can make their way into the Fallopian tubes.

After sex determination, growth continues. Nerves have developed and make many thousands of connections, the fetus begins moving and kicking inside of the mother, and testing its body functions. In fact some fetuses suck their thumbs during gestation. The eyes are sealed closed during this time, due to toxins in the sac around the fetus, and the nose is plugged. Until birth the fetus will not take a first breath. It is during the last trimester, from six to nine months that the baby grows and gains weight. The baby is ready. But having a good weight after birth matter for survival. The skull is not yet closed because trying to pass through the pelvis with a closed skull could kill the mother and the baby.

It is during the birthing process that important things happen. The body induces hormones to ready the lungs of the baby, so that they are functional once it is out. Hormones (oxytocin) tell the mother it is time to give birth and the uterus begins contracting. These contractions weaken the walls of the uterus, and cause them to retract. the size and weight of the skull on the pelvis pushes the back as well. As the uterus contracts the pelvis also opens and the pelvic bone actually cracks open so there is enough room for the skull of the baby to pass. After the uterus is dilated 10cm then it is time for the birthing process. The baby's head is pushed against the uterus over and over, causing bruising, but forcing it to retract. Once the head is through the uterus it must pass the pelvic bone and the vaginal canal. Which can be too small causing it to need to be cut open further. Some times the pelvis is too small meaning the baby needs to be removed via C-section. Once the baby is out the mother will release more oxytocin which will stimulate milk production and the need to nurture.

That is how you go from egg to baby.