Now that we’ve covered the female and male reproductive systems, we turn to what happens when they come together to make a new person. This is fetal development, or, in everyday language, how babies are made.

First, let’s clarify the medical definitions of a few words so we’re all on the same page. An embryo is the early stages of development of a new baby. Whether it’s one cell or many cells, it’s called an embryo until about eight weeks after fertilization. At this point, all of the major organ systems have formed, although they’re not fully functional until later on. From the ninth week onward, the new life is called a fetus. Medically speaking, only after birth does this new life officially become a baby. Not before. Although most people use these terms interchangeably (even doctors will refer to the “baby” growing inside you), we’ll try to stick with the official definitions in this book to avoid confusion. This whole topic is confusing enough as it is.

FERTILIZATION

The first biological step in the development of a new person is fertilization. As we discussed, a sperm swims through the uterus and fallopian tube to look for an egg. 

The most fertile time in the menstrual cycle is the two days leading up to ovulation. So if ovulation occurs around Day 14 of the menstrual cycle, a woman is most likely to get pregnant with intercourse during Days 12-14. 

If the timing is ideal, the sperm will meet the egg just after it bursts out of the ovary. The first sperm to successfully fuse with the egg triggers a chemical reaction that prevents other sperm from binding. Within the egg, the half-set of DNA from the father combines with the half-set of DNA from the mother to produce a full set of new DNA. That’s fertilization: a sperm plus an egg equals a new cell.

At this point, the new cell is called a zygote. This is the first stage in embryonic development. The zygote is a single cell with a complete set of DNA, and it begins replicating right away. The single cell divides to become two, then four, then eight, and so on. Each has its own copy of DNA. Within a few days, there’s a cluster of cells, each a replica of the original. At this point, the embryo is called a morula. All the cells in the morula are the same; none of them have differentiated into other cell types. They each have the potential to become any type of specialized cell within the new body. 

I’m sure you’re already a bit overwhelmed with all the stages the embryo goes through, but these early steps are important to understanding birth control, as we’ll see later.

During those first few days after fertilization, as the new cells are replicating, the embryo drifts down the fallopian tube and arrives in the main part of the uterus. By this time, the cells that were all the same have started to differentiate, that is, they become different from one another. Some cells form an outer layer, some cells form an inner layer. A pocket of fluid develops within the outer layer, pushing the inner cells to one side. Now that we have different cell types, the morula has matured into a blastocyst. This is the stage that implantation occurs at.

Lost yet? Look at the diagram to get an overview of what we’ve just discussed. To summarize:

1. 1. Sperm combines with egg (fertilization)

   2. One cell (zygote) becomes cluster of cells (morula)

   3. Cluster of cells migrates down to uterus

   4. Cells form different layers (blastocyst)

So now we have an embryo floating in the uterus, ready to implant. This next step is critical.

IMPLANTATION

Around Day 21, the lining of the uterus is ready to receive the embryo. Implantation is when the embryo (at the blastocyst stage) gets stuck to the lining of the uterus. Uterus cells wrap around the embryo, and the embryo burrows into the lining of the uterus. Implantation is usually complete 8-10 days after fertilization, or around Day 23 of the menstrual cycle.

Then a whole bunch of stuff happens at once. Because their cells are now touching, the mother and embryo start passing chemical signals to each other. This communication makes both the uterus and embryo shift gears and start developing like crazy.

Let’s start with the embryo. Although some differentiation has already occurred to get to the blastocyst stage, the embryo’s cells start differentiating even more and adopting different roles. Cell layers fold and loop on themselves, forming tubes and discs and more pockets of fluid. The embryo develops different ends, which become the head and backside of the fetus. 

The uterus starts changing too. Up until this point, the embryo has been receiving nutrients from the fluid inside the uterus, but as the embryo grows a greater supply is needed. Thus, a placenta develops. Uterine cells around the embryo develop extra blood vessels to feed nutrients to the newly implanted embryo. Hormones from the placenta tell the brain and ovaries to maintain progesterone production and prevent menstruation. 

All in all, an embryo progresses from a cluster of cells casually travelling down the fallopian tube to a complex arrangement of cell layers embedded within the uterine lining that communicates via chemicals to its mother and starts forming a placenta. As you can see, a lot happens after implantation, and it all happens in a matter of days. But the big picture is that this flurry of activity has triggered changes in the woman that we call pregnancy. 

Now, just a note about conception. This a confusing term. It can refer to fertilization or implantation, depending on who you ask. It can also imply that a new person has formed. We’ll avoid using the word “conception” altogether, since it can be a source of misunderstanding.

FETAL DEVELOPMENT

A note about twins: sometimes multiple eggs are released from the ovaries during ovulation. These individual eggs each have the potential to become fertilized and implant in the uterus. That’s how fraternal (non-identical) twins are produced. Or triplets. Or quadruplets. They all have their own placentas.

However, identical twins occur when a single embryo splits into two. Because the cells are all identical, both clusters develop into identical fetuses. They may or may not share a placenta.

Regardless of how many embryos implant, a fully mature fetus develops within the next 38 weeks, with organs that are completely functional and ready to maintain life independent of its mother. Any defects or deviations in development can be catastrophic, leading to pregnancy loss or congenital abnormalities with a wide range of outcomes. We won’t get into those here – we’ll just assume normal fetal development for the purposes of our discussion.

Sometime later in the pregnancy, the fetus comes out of the uterus. This occurs either via vaginal delivery or C-section. Either way, the fetus becomes a baby as it leaves the uterus. The placenta is no longer needed and is discarded after birth. 

THE BOTTOM LINE

The tricky part, ethically, is this: when exactly does a collection of cells become a new person? It may seem arbitrary, but your answer may determine what type of birth control you’ll be comfortable using. It also has implications for reproductive technology, stem cell research, and pregnancy termination. For many couples, when personhood begins is a big deal.

That’s what the next section is all about.