A new study from Canada suggests that some antibiotics used in the first 20 weeks of pregnancy may significantly increase the risk of miscarriage. This study suggests that current guidelines on antibiotic use during pregnancy may need to be updated. [1,2] First, let’s review the current guidelines.
Pregnancy may increase your risk for some bacterial infections, especially upper respiratory and urinary tract infections. An untreated infection is dangerous for you and your baby, so antibiotics are needed. In fact, about one out of four women will need to be treated with an antibiotic during pregnancy. 
Some antibiotics have been linked to birth defects. The American College of Obstetricians has guidelines based on large research studies to help doctors chose the safest antibiotics to use during pregnancy. The first guideline is that antibiotics should only be used if absolutely needed and then for the shortest amount of time needed. 
According to the guidelines, antibiotics safe to use any time during pregnancy include: 
- Penicillins, including amoxicillin and ampicillin
Sulfonamides and nitofurantoins may increase the risk of birth defects if given in the first trimester, but can be given after the first trimester. These antibiotics are commonly used to treat urinary tract infections. 
The New Study and the Risk of Miscarriage
The current guidelines are primarily concerned with birth defects. The new study concerns spontaneous abortion before 20 weeks of pregnancy. The study was published in 2017 in the Canadian Medical Association Journal. [1,2]
Researchers from the University of Montreal analyzed data from over 180,000 pregnant women from 1998 through 2009. They found 8,702 miscarriages before week 20. About sixteen percent of these women had taken an antibiotic. From this data, the researchers concluded that some antibiotics increased the risk of miscarriage. Depending on the antibiotic, the risk increased from 65 percent to more than double. [1,2]
Antibiotics that increased the risk of miscarriage when taken before week 20 included:
- Azithromycin (Zithromax)
- Clarithromycin (Biaxin)
- Quinolones (Cipro, Noroxin, Levaquin)
- Sulfonamides (Septra, Bactrim, Gantrisin)
- Metronidazole (Flagyl)
Metronidazole is sometimes given for vaginal infection during pregnancy. Tetracycline antibiotics (tetracycline, doxycycline, minocycline) were also found to increase miscarriage, but these drugs should not be used in pregnancy because they are known to cause liver damage in mothers and discolored teeth in children. [1,2]
Antibiotics that did not increase the risk for miscarriage included: [1,2]
The key takeaway from this study is that miscarriage is another thing to ask your doctor about if you need an antibiotic early in pregnancy. Always let any healthcare provider know if you are or may be pregnant before starting any antibiotic. The quinolone antibiotics, which are safe for birth defects, may not be safe for miscarriage. The safest antibiotics during pregnancy include penicillins, erythromycin, and cephalosporins. These drugs should be able to take care of most common infections. [1-3]
- Center for Infectious Disease Resource and Policy, Antibiotics in pregnancy tied to higher miscarriage risk.
- Canadian Medical Association Journal, Use of antibiotics during pregnancy and risk of spontaneous abortion.
- Medscape, Most Antibiotics Safe to Use During Pregnancy.