Last Friday, Mark Zuckerberg announced (on Facebook of course) that he and his wife Priscilla Chan are expecting a baby girl. In the emotional and public post, the CEO also disclosed that Chan had three miscarriages while the couple was trying to conceive.

Miscarriage is a “lonely experience,” Zuckerberg said, because most people don’t talk about it. But speaking with friends who had lost pregnancies and went on to have healthy babies restored the couple’s hope. “We hope that sharing our experience will give more people the same hope we felt and will help more people feel comfortable sharing their stories as well,” he explained.

Nearly 1.7 million likes and more than 117,000 shares later, it’s clear that the couple’s post has raised awareness of miscarriage, and has encouraged others to speak out, too.

Read on to find out more about what we know—and don’t—about this little discussed topic.

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They are more common than you think

“Miscarriages are definitely more common than people realize,” Joshua Klein, MD, a reproductive endocrinologist and fertility specialist at Mount Sinai Hospital in New York City, told Health. Reason number one, he added, is that people simply don’t talk about them. And this can make women who miscarry feel alone and unsupported, Dr. Klein said.

Overall, he says, 25% to 30% of pregnancies end in miscarriage. But because early miscarriages can be mistaken for a late, heavy period—or even a regular period—they often go unnoticed. The percentage of miscarriages among women who actually know they are pregnant is lower, at 10% to 15%, according to the March of Dimes.

The older a woman is, the higher her miscarriage risk. Women in their 20s have a 9% to 17% chance of miscarriage. At about 35, miscarriage risk is 20%, at 40 it’s 40%, and by 45 it can be 80%, according to the American College of Obstetricians and Gynecologists.

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Dad’s age also plays a role

While the effect of a woman’s age on her risk of miscarriage is well known, the age of her partner is also a factor. One large study by Columbia University researchers found a 60% higher risk of miscarriage for fathers 40 and older, regardless of the mother’s age. A European study from the journal Human Reproduction showed a greater likelihood of miscarriage in women 35 and older if their partner was at least 40. Yet another investigation, from the American Journal of Epidemology in 2005, showed a 27% increased miscarriage risk for men 35 and older.

Two or three in a row are cause for concern

Once a woman has experienced two or three miscarriages in a row, Dr. Klein says, she is considered to have recurrent pregnancy loss. At this point, it’s a good idea to see a fertility specialist, who can determine if medical issues are interfering with her ability to carry a pregnancy. The short list of possible culprits includes blood clotting disorders, structural problems of the uterus, immunologic issues, and genetic factors, he says.

Problems with blood clotting or immunologic factors can be treated with medication, while anatomic issues can be treated with surgery, according to Dr. Klein. And while it’s not possible to “treat” genetic factors related to miscarriage, he added, it is possible for couples to undergo in vitro fertilization with eggs and sperm screened with genetic testing.

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Mismatched chromosomes are usually to blame

Most of the time, miscarriage is due to an embryo not having the right number of chromosomes, according to the American College of Obstetricians and Gynecologists. Egg and sperm should have 23 chromosomes each. But too many or too few chromosomes—in egg, sperm, or both—produce an embryo that probably won’t survive.

Older women have more miscarriages because their ovaries produce more abnormal eggs as time goes on, Dr. Klein says. That’s why some women who aren’t ready to have children in their younger years may opt to freeze their eggs. And sperm can also be more prone to genetic mistakes as men age.

There's no need to wait three months to try again

The conventional medical wisdom has been that women need to wait several months after a miscarriage to start trying to get pregnant again. But getting pregnant soon after a miscarriage doesn’t mean you are at greater risk of losing that second pregnancy, Dr. Klein says. Instead, he explains, women can wait for a month to allow their menstrual cycle to normalize, and start trying again. Waiting too long is an especially bad idea for older women, he adds, for whom every cycle counts. “You may be hurting yourself by waiting, and you’re not helping yourself.”

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