Despite how foreign they can look on your body, stretch marks (or striae) are a normal part of pregnancy: Half of all moms-to-be can expect to find these rippled stripes on their skin.
What's the cause?
"Stretch marks are an odd entity," says dermatologist Alan Rosenbach, M.D., a clinical assistant professor at Keck School of Medicine, University of Southern California in Los Angeles. "Considering how common they are, we don't know much about their cause, though some suspect the reasons are at least partly hormonal."
Pregnancy stretch marks often appear at about 25 weeks, though many women notice them in the first trimester, before they've gained any appreciable weight. That may be because genes also play into the equation. "It's not weight gain as much as genes that raise risk," Rosenbach says.
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Who gets them?
A Stanford University study found the odds of getting stretch marks were higher when women already had striae on their breasts or thighs—usually since puberty—or had a family history of such marks. Almost half who had striae gravidarum (stretch marks associated with pregnancy) had mothers with them. Having darker skin also was a significant risk factor.