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Actress Gabrielle Union's IVF Struggles: What Causes Miscarriage?
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Actress Gabrielle Union has had "eight or nine miscarriages," according to an essay published in her new book.

In the book, Union, who is 44, described her struggles with fertility and in vitro fertilization (IVF), according to People magazine.

"For three years, my body has been a prisoner of trying to get pregnant — I've either been about to go into an IVF cycle, in the middle of an IVF cycle, or coming out of an IVF cycle," Union wrote in the book, "We're Going to Need More Wine" (HarperCollins, 2017). "I have had eight or nine miscarriages," she wrote. [6 Myths About Miscarriage]

Up to 20 percent of known pregnancies (in other words, when a woman knows she is pregnant) end in miscarriage, according to the Mayo Clinic. The actual number of miscarriages, however, is probably higher, because many occur very early in pregnancy, before a woman knows she is pregnant, the Mayo Clinic says. But what causes miscarriages? And what factors raise a woman's risk?

The most common cause of miscarriage — which is the spontaneous loss of a fetus before the 20th week of pregnancy — is genetic problems that arise when the fetus is developing, according to the National Library of Medicine. (After the 20th week, the spontaneous loss of a fetus is considered a stillbirth.) These faulty genes are rarely inherited from the mother or the father; rather, the errors occur when the cells of an embryo or a fetus are rapidly dividing. This is especially the case for mothers who have had multiple miscarriages, according to a 2012 study of nearly 500 women that was published in the journal Human Reproduction.

These types of miscarriages have nothing to do with the mother's health or activities; rather, they result from events that occur by chance, according to the Mayo Clinic.

Doctors refer to two or more consecutive miscarriages as "recurrent pregnancy loss," according to UCLA Health. Recurrent pregnancy loss sometimes may be caused by genetic abnormalities in the mother, UCLA Health says, but in more than half of the women with the condition, the cause is unknown. A study from March, published in the journal Mitochondrial DNA Part A, found that recurrent pregnancy loss may be linked to genetic mutations in a woman's mitochondrial DNA. (Mitochondria are the energy powerhouses of cells and have their own DNA separate from the rest of the cell.)

Women with recurrent pregnancy loss have a higher risk of miscarriages, according to the Mayo Clinic.

Certain health conditions can also raise a woman's risk of miscarriage, including uncontrolled diabetes, infections, hormonal problems, uterine or cervical problems, and thyroid disease, according to the Mayo Clinic.

Age is also a risk factor for miscarriage: A 35-year-old woman has about a 20 percent risk of having a miscarriage; a 40-year-old woman has about a 40 percent risk; and a 45-year-old woman has about an 80 percent risk, the Mayo Clinic says.

Smoking, drinking and using drugs also increase a woman's risk of having a miscarriage, according to the National Library of Medicine.

Original article on Live Science.