What it is: An infection caused by a single-celled parasite known as Trichomonas vaginalis. Estimated to affect 80,000 pregnant women each year, trich is the most common STD among women.
Why it's dangerous: Trichomoniasis infection during pregnancy is linked to a higher risk of premature delivery and to preterm premature rupture of the membranes (PPRM), Dr. Nyirjesy says. It can also make you more susceptible to other STDs, like chlamydia and HIV, if you're exposed to them.
Should you be tested? Only if you experience the symptoms: yellowish, greenish, or grayish vaginal discharge, often accompanied by an unpleasant odor. "There's no evidence that treating symptomless trich lowers your risk of preterm delivery, and even some evidence that it may actually increase it," Dr. Nyirjesy explains.
Treatment: Both you and your partner will be treated with Flagyl. Even if your partner doesn't have symptoms -- and most men don't -- he'll need to be treated so you can avoid reinfecting each other. You'll also have to abstain from sex until you've both completed treatment.