What it is: An STD caused by the bacterium Chlamydia trachomatis, which, if untreated, can spread into the uterus or fallopian tubes and cause fertility-threatening pelvic inflammatory disease. An estimated 2.8 million Americans are infected each year -- including 200,000 pregnant women -- according to the CDC. Even scarier, about 75 percent of those women don't have symptoms like vaginal discharge or pain while urinating, so they have no clue they're infected.
Why it's dangerous: You can pass chlamydia to your baby during delivery, and she can develop pneumonia as a result. This can be fatal for a newborn, says Miriam Greene, MD, an ob-gyn at New York University Medical Center.
Should you be tested? Absolutely -- the CDC recommends that all pregnant women be screened at their first prenatal visit.
Treatment: If you've got chlamydia, both you and your partner will be treated with the antibiotic Zithromax, says ob-gyn Wendy Wilcox, MD, medical director at Montefiore Medical Center, in the Bronx, New York. You should abstain from sex until you've both completed treatment, so you don't get reinfected. Your doctor should also repeat the test during your third trimester, to make sure you're not infected during delivery.