Fertility 101

Reproduction is complicated, but at its most basic core, a few elements are absolutely required for a healthy pregnancy and live birth to take place:

1. A high quality and genetically normal egg
2. Healthy sperm
3. At least one functional fallopian tube
4. A uterus that is ready and able to hold a pregnancy
5. Hormonal support for each event

To over simplify, a healthy egg and a healthy sperm are both needed. They need a place to meet (the fallopian tube) so that fertilization can happen. The fertilized egg must be carried through the fallopian tube to a healthy uterus which needs to be able to support a pregnancy. Each event must be supported by the presence of the right hormone, in the right amount, at the right time.

If any element is unhealthy or missing, a healthy pregnancy cannot take place.

It’s impossible to understand the process of reproduction without understanding the role of hormones and the menstrual cycle. If any one hormone is off, certain events just can’t take place.

Picture an orchestra. The hormones that control the menstrual cycle are the conductor. If the conductor is late, misses cues, or doesn’t show up at all, the whole concert falls through. In this section, we’ll explain what ideally takes place during the menstrual cycle to make reproduction happen.

A quick overview: there are different sequences of events that happen with hormone levels, with the ovaries, and with the lining of the uterus during a typical 28 day cycle. From a timing perspective within the average 28-day menstrual cycle, these sequences can be thought of in three main chunks: the start when the period begins, the follicular phase during which an egg matures in an ovary, and the last 12-14 days called the luteal phase.

For cycles that are a different length than 28 days, the last 12-14 days are relatively fixed as the luteal phase, and the remainder of the time is the follicular phase - for example, if the cycle is 35 days, then the first 21 days is the follicular phase, and the last 14 days are the luteal phase. Ovulation happens at the end of the follicular phase, and the exact cycle day depends on the length of the cycle.

The Menstrual Period

Each month, hormones prepare the uterus to receive a fertilized egg and to allow for implantation of an embryo. When implantation doesn’t happen, the lining of the uterus that has thickened during the last cycle, sheds in what’s known as menstruation or a menstrual “period.” The first day of menstruation is considered day 1 of the menstrual cycle.

The Follicular Phase

Simultaneously with the menstrual period ("menses" as seen below), follicles begin to grow.

Follicles are structures on the surface of a woman’s ovaries. And importantly, follicles contain a woman’s microscopic, immature eggs. During the follicular phase, a hormone called follicle stimulating hormone, or FSH, is released from the pituitary gland in the brain. FSH tells these follicles to grow.

As FSH supports the growing follicles, the follicles release estrogen. And estrogen prepares the uterus for a pregnancy. The cells that line the uterus are collectively called the endometrium. The endometrium must grow quickly in order for successful implantation to occur. The thickening of the lining of the uterus, or endometrium, is dependent on estrogen—and in the right amount.

On day 7 or 8 of the cycle, one dominant follicle (or occasionally two) is selected for continued growth—the rest of the follicles do not mature and instead degenerate.

Ovulation

Around the middle of the cycle, the process of ovulation begins. The rise in estrogen produced by the dominant follicle causes a hormone called luteinizing hormone, or LH, to be released. LH helps the egg in the dominant follicle to continue to “mature.”

In overly simplistic terms, the most important part of this maturation process is the egg shedding half its DNA blueprint to make space for the sperm DNA. The follicle then ruptures and the mature egg is released. Once the egg is released, or what’s known as ovulation, the luteal phase begins.

Luteal Phase

When the egg ovulates, the fallopian tubes come into play. The fallopian tubes essentially vacuum the surface of the ovary and bring the now-mature egg into the tube to meet the sperm.

Of course, that means there needs to be sperm waiting in the fallopian tube to meet the egg when it is ovulated or very soon thereafter. The egg usually degenerates within 24 hours after ovulation. For now, let’s assume the sperm is where it needs to be at the right time, and we’ll cover the details on how that happens soon. (Just know, this probably means intercourse happened in the 72-hour window leading up to ovulation, though sperm can live up to 5 days in the reproductive tract.)

When the follicle that contained the egg has ruptured, it transforms into something called the corpus luteum. The corpus luteum releases estrogen and progesterone which are both essential for implantation in the uterus a few days later.

Meanwhile, in the fallopian tube, many sperm attack the egg and try to pierce its outer layer. When one sperm penetrates the egg, “fertilization” happens.

Over the next few days, the cells of the fertilized egg divide and create an embryo. After about three days of growth, the fallopian tube transports that embryo into the uterus.

In the uterus, the embryo continues to grow, becoming what’s known as a blastocyst. It then “hatches” and starts the process of implantation. The endometrium sends signals to the embryo, pulling the embryo into the lining. The embryo is pulled in, sticks, and invades the endometrial lining.

When implantation happens, the hormone human chorionic gonadotropin, or hCG, is released. This hormone might sound familiar because it’s the one that’s measured in pregnancy tests. HCG keeps the corpus luteum (the ruptured empty follicle where the egg used to be) alive so that it continues to release estrogen and progesterone. Estrogen and progesterone support the endometrium during this second half of the menstrual cycle, and in turn, the endometrium supports the growing embryo until the placenta has formed.

If any tiny piece of this process is missing, there won’t be a pregnancy. In that case, the corpus luteum goes away 12 or 14 days after ovulation. That means the hormonal support for the endometrium disappears, and we are back where we started—the menstrual period—where much of the endometrial lining sheds in a woman’s period, starting the whole cycle again.

It’s important to note that these are just the steps leading to the beginning of a pregnancy—there are a lot of other things that must go right to maintain this pregnancy and eventually lead to a live birth. Heartbreakingly, not all conceptions lead to a live birth; we have a course on the topic of miscarriage you can find here.