Fertility 101

The most simple way that reproduction can go wrong is when intercourse isn’t timed so that sperm is where it needs to be before the time of ovulation.

Sperm can live up to five days, but generally speaking, sex should happen in the three days leading up to ovulation to make sure sperm are present in the fallopian tubes at the time of ovulation.

You might be surprised how often people think they’re tracking their cycle to perfect this timing, but still get it wrong—we’ll share more later on about how to optimize this.

Of course, the presence of an egg is non-negotiable in this process. This requires a functioning ovary. There are a few broad buckets where an egg won’t be available.

• Ovulation isn’t happening
• There aren’t eggs, or follicles, left in the ovary

Ovulation Isn’t Happening

If ovulation isn’t happening, then mature eggs are not being released into the fallopian tubes, so the rest of the reproductive process can’t happen.

This is often the case when there is a hormonal imbalance, for example, with women who have polycystic ovary syndrome (PCOS). The microscopic follicles in the ovary aren’t getting hormonal signals to grow and develop. Without a dominant follicle growing during a cycle, there won’t be a mature egg released during ovulation.

Depleted Egg Reserve

Women are born with all the eggs they’ll ever have, so if the stockpile of eggs has been completely depleted, the process can’t happen. By the time a woman reaches menopause (at an average age of 51), her ovaries will no longer have any microscopic, egg-containing follicles left. It’s also possible that this could happen before that age because of damage done to the ovaries or primary ovarian insufficiency. It’s pretty rare that a woman of reproductive age would no longer have follicles or eggs in her ovaries as only about 1% of women under age 40 undergo premature menopause.

In this case, even if all the right hormones are released, the ovaries won’t listen. So even if a menopausal woman is given high levels of FSH, it won’t jump-start her ovaries.

Unfortunately, the majority of ovulated eggs are not capable of producing a healthy pregnancy. And the likelihood that a given egg has all the right stuff to produce a healthy baby decreases as a given woman ages—this is the main reason pregnancy rates decrease with a woman’s age and miscarriage rates increase.

To produce a healthy baby, an egg must be of high quality. There are a number of factors that contribute to egg quality, but likely the most important factor is an egg’s ability to mature and develop with the correct number of chromosomes. For the moment just know that being “chromosomally normal” or “euploid” is essential for a healthy pregnancy—this generally becomes less frequent as a woman ages.

Below, we present the results of two studies that examined the relationship between age and the percentage of euploid embryos. While each study shows slightly different percentages for specific ages, both reveal a similar pattern: the percentage of euploid embryos decreases as age increases.

Chromosomes carry all of our genes, and it’s hard to overstate how important it is for the viability and health of future children that an egg—and resulting embryo—have the correct number of chromosomes.

Since this is all probably pretty abstract to most of us, it’s easiest to understand through an analogy. Think of DNA as the full book shelf (or, if you remember this far back, an entire encyclopedia set). A chromosome is a single book. Genes are the pages in those books, and mutations are the letters on those pages. If all of us are walking, talking encyclopedia sets, we each have 46 books, or chromosomes, 23 of which are genetically contributed from an egg and 23 contributed from the sperm.

During egg maturation, the crucial process of meiosis takes place—an egg needs to shed half its DNA blueprint, going from 46 to 23 chromosomes, making room to combine with the sperm’s DNA.

In a perfect scenario, division happens evenly. But, as women age, it’s more likely that an uneven division will occur, leaving a mature egg with an extra or missing chromosome. In that case, the resulting embryo will have a number other than 46—this will either result in an embryo that can’t implant, that results in an early miscarriage, or a baby with a chromosomal abnormality like Down syndrome.

An egg—and embryo—that is chromosomally normal is called euploid, and one that is chromosomally abnormal is called aneuploid.

If an egg doesn’t have the right number of chromosomes, then it’s not considered high quality enough to lead to a healthy pregnancy.

Sperm can also be a cause of aneuploidy, though this is much less common than aneuploidy in the egg.

The number of chromosomes in an embryo is not the only consideration as an aneuploid embryo may have a total chromosome count of 46, but this count could result from an abnormal combination, such as one monosomy (missing one chromosome) and one trisomy (having an extra chromosome).

If the fallopian tubes are blocked or damaged in any way, natural reproduction will be impeded. For example, if a fallopian tube is damaged by scar tissue, it won’t be able to vacuum eggs off of the ovary. Likewise, if the tube is blocked, there will be no place for egg and sperm to meet, so fertilization can’t take place naturally. Abnormalities of the fallopian tube, such as hydrosalpinx, can also make implantation difficult.

Finally, if fertilization has happened in the fallopian tube, the tube needs to then transport that embryo back into the uterus—otherwise, the pregnancy will take place in the tube itself, which is life-threatening and known as an ectopic pregnancy. Tubal function could be compromised for a number of reasons, including smoking, endometriosis, or previous infections like sexually transmitted infections.