Essay:Prenatal development

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I'm going to begin writing a guide to prenatal development, day-by-day. Kind of like a pregnancy calendar, but with the intent of debunking pro-lifers' claims like this one from an old tract available here:

We know the baby's sex within 5 days of conception. We know that at 18 days the heart is developing and at 19 days, the eyes. At 24 days, the heart is beating. At 28 days, arms and legs are forming. At 42 days, the skeleton is complete and the baby has reflexes. At 43 days, brain activity begins!

NOTICE: This is not supposed to give pregnancy suggestions or serve for any medical use. I'm trying to find citations when I can, but sometimes getting numbers to match up means a small bit of PIDOOMA. I'll try to keep such instances to a minimum, and so I will provide citations whenever possible. If an un-cited number is here and you don't believe me, just look it up yourself and tell me about what you find. If you come here and you don't see every day on here, it's because I'm still working on it: it may take me 9 months to write an article chronicling 9 months! Okay, it'll probably take less than that, but don't expect this to be an overnight thing. The Heidelberg Kid (talk) 01:45, 18 February 2012 (UTC)

First Trimester[edit]

Week 1[edit]

Day -1: Insemination[edit]

Having spent the past hour or two[1] swimming, some 250 sperm out of 250 million contestants[2] manage to reach the prized oocyte. Only one of the sperm is able to successfully penetrate through the thick protective jellylike zona pecullida which formed when the oocyte was first maturing, a process that takes another ~7 hours to complete. Having managed to break digest his tunnel, the winning sperm's head merges with the oocyte's membrane, the tail comes off, and the oocyte releases chemical sacs to make the zona indigestible to sperm. The drawbridge is up.

Day 0: Conception[edit]

The nucleus of the sperm begins gradually waltzing towards the egg's nucleus. Due to the enormous size of the oocyte, the journey takes about 12 hours[3] to make. As the sperm's nucleus approaches, it begins to disintegrate due to enzymes in the egg's cytoplasm. The nuclear wall fully digested, the chromosomes themselves enter into the egg's nucleus. This is the moment of conception.

Day 1: Cleavage[edit]

Almost immediately after fertilizing, the oocyte, or zygote as it is now called, begins to duplicate. It takes another half of a day in order to make the first multiplication[4] Another half of a day or so passes in order to replicate again, this time into four cells. The cells also produce a substance called early pregnancy factor so that the immune system will not attack the "foreign" cells.

Day 2: Compaction[edit]

The day begins as the four blastomeres divide to become eight cells, still together. Up until now, it would have been entirely possible for one of the cells, not tightly joined to the others, be able to separate itself from the rest of the cells and develop on its own, resulting in the birth of twins if both parties survive. However, this no longer is a possibility, as the cells begin to make proteins in order to hold themselves together in what is called a "tight cell junction". Twinning will not be possible for a few weeks, due to what will happen tomorrow.

Day 3: Differentiation[edit]

This day, the group of 8 compacted cells divides again to form a solid ball of 16 cells called a morula, from the Latin word for "mulberry". Most of the cells exist on the one-cell-thick surface of the ball, but a few take up residence on the inside. Up until this point, all of the cells descended from the zygote were totipotent[5]. However, the cells begin to differentiate, or specialize. In this case, as always, the cells' fates are determined by their location, which controls their chemical environs. Here, the cells on the surface of the ball are exposed to chemicals in the oviduct which indicate to become placental cells, while those on the inside are not exposed and so will not become a placenta, but rather the inner cell mass which will become the baby if no complications arise.

Day 4: Cavitation[edit]

The cells are dividing again now, but we can no longer keep track of how many there are, now that some have been effectively shut off from our view. The cells on the surface of the morula begin to secrete a fluid which turns the morula, solid until then, into a hollow chamber one cell thick except for a small bump on the inside that is the inner cell mass. The morula, no longer resembling the mulberry of which it was named, is now called a blastocyst, and has at this point traveled through most of the length of the Fallopian tube. With the formation of the blastocyst, it is now possible to see which side of the inner cell mass will become the baby's back, and which side the chest.

Day 5: Hatching[edit]

Remember the zona pecullida that surrounded the first few cells? Well, the blastocyst has been and still is inside of there. The size of the blastocyst now is the same size of the oocyte from five days ago. If the blastocyst is to be able to grow, or even implant in the womb tomorrow, then it will need to break free of what once was a protective cage but is now an oppressive prison. On this day, the blastocyst manages to break through the zona, escaping it to continue its journey alone. The zona will be discarded, while the blastocyst is currently searching for fertile ground to settle in.

Day 6: Implantation[edit]

The journey so far for the blastocyst has been very long and hard. If there were 200 initial oocytes that were near sperm, only 68% of them will succeed at their next task. Failure would mean that the blastocyst will not get access to any more resources. The initial yolk sac in the oocyte which nourished the cells up to now is nearly gone. But if nothing is done, the cells will starve and the lifeless hollow ball will leave the chamber of the uterus in a period, and no pregnancy would have occurred. Most of the embryos-not-to-be had severe genetic abnormalities, such as the wrong number of chromosomes, which hindered their attempts at survival.[6]

Special adhesive molecules manage to "glue" the blastocyst onto the velvety endometrium which has the job of supplying resources to the embryo-to-be until a more permanent method can be arranged. The cells on the surface of the ball share the food with those in the inner cell mass: sharing food will be the main purpose of the surface, as we will see tomorrow.

Week 2[edit]

Day 7: Placentation[edit]

The placenta is an organ that is of no use once a baby is born, but is vital to the survival of the embryo. The placenta has the job of transferring nutrients from the mother to the embryo. Out of all of the cells that a pluripotent stem cell can become, they cannot become cells found in the placenta. Our second round of differentiation begins, as the placental cells "decide" whether to become part of the actual placenta or part of the chorion which serves as a "membrane" between mother and child. The inner cell mass is also differentiating: the cells inside are no longer pluripotent: some of them are part of the hypoblast and others part of the epiblast.

Day 8: Amniogenesis[7][edit]

The embryo has now managed to successfully implant in the womb, now surrounded on all sides by uterus. Of course, a lot of the embryo's actions over the past few days were to allow it to just survive long enough to set up shop: hatching, implanting, and placentating. Of course, for all its efforts, the embryo still is at risk of being shed in the menstrual cycle. Obviously, an embryo cannot survive in a bloody mess like that. Today, the embryo produces a hormone referred to as hCG, which will interfere with the menstrual cycle and is typically detected by most pregnancy tests.

Now that the initial tasks of being able to survive more than month are done, there is much practicing the future fetus will need to do. Once a baby is born, it will have literal seconds in order for its body to recognize the placenta is no longer going to nourish you, and you will need to breathe in your own oxygen and move around on your own power. While the outer layer of the old blastocyst continues making the placenta, some members of the epiblast, called amnioblasts, begin secreting a fluid which will begin to flatten the rest of the inner cell mass like a pancake by the two-week mark. The fluid is called amnion, and the amnioblasts will form the amniotic sac, in which the fetus will someday be able to perform acrobatics and "learn" to breathe and move months before such knowledge would ever come to good use.

Day 9: Vitellogenesis[8][edit]

Sorry it seems like it's taking so long, but I think one a day is fair. At least one guy is rooting for me to take 9 months to do this, and quite frankly, it can get a bit tedious. No, I'm not lazy, and I'm not going to shirk out of doing this. Godot for one thinks this is a great idea, and I think most people would be glad to have me out of their hair for some time. The Heidelberg Kid (talk) 01:36, 26 February 2012 (UTC)


  1. The journey from vagina to oviduct is about five inches long.
  4. This can be inferred due to the differences in the second through fourth images from the left of the photo, and the earlier reference regarding the merging of the pronuclei.
  5. Also sometimes referred to as "omnipotent"
  7. A word I made up to describe this, meaning "formation of the amniotic sac"
  8. Another word I made up, are there real words for these? Refers to formation of the yolk, from the Latin for yolk. Because Latin makes it look like what you're talking about is advanced and sophisticated.