If I had to come up with a list of some of the most confusing health topics, fertility would be right up there at the top. It’s a huge part of the health of any person with a uterus, regardless of whether or not they intend to have kids. But before a person actively starts trying to get pregnant, most people’s understanding of female fertility comes from sub-par high school health classes and one’s annual checkup with their OB/GYN—leaving a lot of room for questions.

To that end, we polled Well+Good’s Instagram audience earlier this month to ask them what they found the most confusing about fertility. And while readers got many of the basic facts right—like how fertility in women typically declines after age 35, and that our hormone levels fluctuate throughout the month—people asked a lot of the same (or similar) questions. There were so many common themes, in fact, that we decided we had to take these pressing Q’s to top gynecologists to get some much-needed A’s. Consider this your 200-level course on female fertility.

1. What can cause infertility?

The “foundation” of successful fertility, says Stephanie McClellan, MD, chief medical officer at Tia, is for an egg to be fertilized by a sperm. “To accomplish that, number one, a [person] has to ovulate—meaning that with some regularity, they produce in their ovaries an egg—in a supportive hormone environment that allows for fertilization and implantation of the fertilized egg in the uterus,” she says. But there are all kinds of things that can come up from both partners that can impact this process and prevent pregnancy, she says.

One of the biggest roadblocks is ovulation dysfunction, says Alyssa Dweck, MD, OB/GYN, a gynecologist in New York City—if the ovaries don’t release an egg, the sperm has nothing to fertilize. One in four infertile couples deals with this issue, per the Mayo Clinic. It can be caused by things like polycystic ovarian syndrome (PCOS) or premature ovarian failure (when a person’s eggs age and die abnormally soon).

Some other things on the female side that can cause infertility, according to Dr. Dweck: blocked fallopian tubes (which prevents sperm from meeting an egg), hormone imbalances, infections of the uterus or other reproductive organs, and structural issues like fibroids, polyps, or scar tissue.

On the male side, low sperm count (meaning that semen contains an abnormally low amount of sperm) and motility (meaning sperm can’t swim well or at all) are common issues, says Dr. McClellan. Issues with sperm can make fertilizing even the healthiest egg much more difficult—if a sperm can’t swim well, for example, it has less of a chance to make it through the vagina, cervix, uterus, and fallopian tube to meet up with an egg to fertilize.

2. When is the “fertile window” during your cycle, and how long does it last?

“The ‘fertile window’ is when you are most likely to become pregnant during your menstrual cycle,” says Gillian Dean, MD, the senior director of medical services at Planned Parenthood Federation of America. This is around the time you ovulate, when your ovaries release an egg into your fallopian tube. An unfertilized egg can survive in the fallopian tube for 12-24 hours, and sperm can survive for around six days in the uterus and fallopian tube. “This means that you are most fertile for about seven days of every menstrual cycle: the five days before you ovulate, the day you actually ovulate, and the day or two after you’ve ovulated,” she says.

However, everyone’s exact fertile window is different—some people have super regular cycles and they ovulate at the same point each month, while others have irregular cycles that make it a bit harder to pinpoint when ovulation happens (and thus, what the fertile window would be). For example, while a fertile window usually happens between periods, “for some people, especially those with short cycles or irregular bleeding, the fertile window can happen during a bleeding day,” Dr. Dean says. That’s why it’s technically possible to get pregnant if you have unprotected sex at any point during your cycle, even during your period.

That’s why it’s technically possible to get pregnant if you have unprotected sex at any point during your cycle, even during your period.

Ovulation typically happens about 14 days before your next period, says Dr. Dean; most people can figure out when they ovulate by tracking their period using apps or the calendar method (mapping out your cycles on a calendar and then doing math to estimate the fertile window). It’s even more accurate, Dr. Dean says, if you add on extras like tracking your temperature or vaginal discharge, which can also help indicate when ovulation happens. These extras, she says, should be done with the help of your health practitioner to better understand and interpret this data.

Get a first-hand look at what a fertility test can look like…if you get one in Kindbody’s van: 

3. What do irregular periods mean for female fertility?

While everyone’s periods are a bit different from each other, generally a period is considered normal or healthy if it occurs every 21 to 35 days and lasts for two to seven days. An irregular period, says Dr. Dean, is when a person misses their period, experience heavier or lighter bleeding than usual, and comes with unpredictable timing each month. “If many of your periods are irregular, talk with your provider to make sure everything is okay as this could be a sign of an underlying health problem that could impact your fertility,” she says.

For example, irregular periods can be a signal of erratic ovulation, says Mary Jane Minkin, MD, a clinical professor of obstetrics, gynecology, and reproductive sciences at Yale School of Medicine—which, as mentioned above, can impact a person’s ability to get pregnant.

“In addition, irregular or absent flow might be due to thyroid irregularity, elevated prolactin level—a hormone secreted by brain [that can stop ovulation]—or even an eating disorder or excessive exercise or stress,” says Dr. Dweck. Addressing the underlying issue with your OB/GYN can help, she says.

4. Does hormonal birth control impact a person’s long-term fertility?

While you’re on birth control, your fertility is impacted—that’s the whole point. Your ovulation is either suppressed (as with the pill) or your uterine and cervical environment are changed to make them inhospitable to sperm (as with the IUD or other forms of long acting reversible contraception), says Dr. McClellan. But once you stop using hormonal birth control, Dr. Dean says, your fertility should not be affected at all. “As long as there are no other factors impacting your fertility, you can get pregnant no matter if or how long you’ve used birth control in the past,” she says.

However, the reason why a person is taking hormonal contraception might impact their fertility, says Dr. Dweck. For example, many people with irregular cycles, endometriosis, or hormonal imbalances like PCOS might be prescribed hormonal birth control to help manage their symptoms. Those underlying conditions can all impact fertility, so people with those issues might have trouble getting pregnant if they go off the pill or get their IUD removed. Plus, “age impacts fertility,” she adds—people with uteruses experience a decline in their fertility starting around age 35. So a person who has been on the pill for years might have trouble getting pregnant once they go off of it, but it’s not necessarily because of the pill, it’s because they’re just older and thus potentially less fertile to begin with.

5. Why do women (and people with uteruses) go through menopause but men don’t?

From a biological standpoint, “our ovaries are designed to stop working at some point,” says Dr. Minkin. “We are born with all the eggs we will have—and each [menstrual] cycle we keep maturing eggs and ovulating—but we are depleting our supply as we go along. And when all the eggs are gone, we are menopausal.” Meanwhile, men and people with testicles keep making sperm and testosterone well into their 70s, she says.

“We are born with all the eggs we will have—and each [menstrual] cycle we keep maturing eggs and ovulating—but we are depleting our supply as we go along.”— Dr. Minkin

“Menopause for women is like falling off a hormone cliff,” adds Dr. McClellan. “You have hormones and then all of a sudden, you don’t. You literally fall the cliff. For men, what we call ‘andropause,’ it’s more like rolling down a gentle hill and men are capable of reproduction into very advanced age.”

This sounds…pretty terrible, especially given the impacts of this hormone loss—hot flashes, lower sex drive (and painful sex), bone density loss, and other health effects. But menopause isn’t all bad, says Dr. McClellan, thanks to the “Grandmother Effect.” People can die during childbirth or immediately afterwards due to complications like blood clots and infections; while these outcomes are generally rare today, they were very common in the centuries before modern medicine. Those risks to mom and baby are even greater as a woman gets older, says Dr. McClellan.

Thus, “if you stop reproductive opportunity or make it less likely to occur… this might be, in fact, a biological gift,” says Dr. McClellan. Women not only survive their pregnancies and deliver healthy babies but also live to take care of those children, she says. Plus, “as women are relieved of their primary duties of mothering, they are available to apply these nurturing experiencing tendencies to the broader community,” she says—what’s known as the “Grandmother Effect.” (This effect is also supported by some anthropological data.)

Speaking of fertility, we asked OB/GYNs to answer some of the most common questions they get about periods. And here’s why fertility is about more than just your ability to have a baby.