Antibiotics in Pregnancy
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Direct answer
Antibiotics in pregnancy are chosen by weighing a drug's fetal risk against the harm of untreated maternal infection. Penicillins, cephalosporins and erythromycin (not the estolate salt) are generally safe, whereas tetracyclines, aminoglycosides and sulphonamides carry named fetal or neonatal toxicities. Benzathine penicillin G is the only recommended treatment for syphilis in pregnancy, so a penicillin-allergic woman needs desensitisation rather than a substitute drug.
What you must remember
- Safe mainstays: penicillins, cephalosporins and erythromycin are the usual choices, and azithromycin is preferred for chlamydia in pregnancy.
- Tetracyclines: avoided throughout pregnancy because they cause enamel hypoplasia, discolouration and inhibition of fetal bone growth.
- Aminoglycosides: streptomycin and related drugs cross the placenta and can cause fetal eighth nerve ototoxicity.
- Sulphonamides: avoided near term because they displace bilirubin from albumin and risk neonatal kernicterus.
- Chloramphenicol: avoided near term because of grey baby syndrome in the newborn.
- Trimethoprim: a folate antagonist, avoided in the first trimester because of neural tube defect concern.
- Nitrofurantoin: avoided near term because it can cause neonatal haemolysis, particularly with G6PD deficiency.
- Metronidazole: acceptable and standard for bacterial vaginosis and trichomoniasis in pregnancy.
A drug-class approach to fetal risk
Antibiotic choice is easiest to recall by class, because each class carries a distinct fetal mechanism and a specific gestational window. The verdict often depends on when in pregnancy the drug is given, not only on whether it is given.
| Drug class | Usual status | Named concern |
|---|---|---|
| Penicillins, cephalosporins, erythromycin | Safe, first choice | Minimal; azithromycin preferred for chlamydia in pregnancy |
| Metronidazole | Acceptable for BV and trichomoniasis | Older caution in the first trimester |
| Nitrofurantoin | Avoided near term | Neonatal haemolysis, especially G6PD deficiency |
| Trimethoprim | Avoided in first trimester | Folate antagonism and neural tube defects |
| Sulphonamides | Avoided near term | Neonatal kernicterus |
| Tetracyclines | Avoided | Teeth and bone effects in the fetus |
| Aminoglycosides | Avoided unless essential | Fetal eighth nerve ototoxicity |
| Fluoroquinolones | Avoided as a precaution | Cartilage concern in animal studies |
Most of the harm is tied to a specific window or mechanism, so the gestational age of exposure matters as much as the drug name. When a drug is avoided, the infection should still be treated with the safest effective alternative rather than left untreated, because untreated maternal sepsis is itself a major threat to the fetus.
Where examiners set the trap
Antibiotic questions usually turn on one named fetal or neonatal toxicity.
- Trap: Avoiding all antibiotics to protect the fetus Correct: untreated pyelonephritis or syphilis harms the fetus more, so the safest effective drug is used.
- Trap: Choosing doxycycline for chlamydia Correct: tetracyclines are avoided, and a single dose of azithromycin is preferred.
- Trap: Assuming only first-trimester exposure matters Correct: sulphonamides and nitrofurantoin matter near term, when kernicterus and haemolysis are the concern.
- Trap: Substituting another drug for penicillin in syphilis Correct: benzathine penicillin is the only recommended treatment, so the allergic woman is desensitised.
Frequently asked questions
Which antibiotic is the only recommended treatment for syphilis in pregnancy?
Benzathine penicillin G is the only recommended treatment for syphilis in pregnancy, because penicillin is the agent proven to prevent congenital syphilis. A woman with a penicillin allergy should be desensitised rather than switched to another antibiotic.
Why are tetracyclines avoided in pregnancy?
Tetracyclines such as doxycycline are avoided in pregnancy because they affect developing fetal teeth and bones, causing enamel hypoplasia, discolouration and reduced bone growth. For chlamydia in pregnancy, azithromycin is chosen instead.
Why is nitrofurantoin stopped near term?
Nitrofurantoin is avoided near term because it can cause haemolytic anaemia in the newborn, particularly where G6PD deficiency is present. Cephalexin is a common alternative for urinary infection when nitrofurantoin must be avoided.
Which antibiotic is used for intrapartum prophylaxis in group B Streptococcus carriers?
Intravenous benzylpenicillin is the usual intrapartum prophylaxis for women colonised with group B Streptococcus, given in labour to reduce early-onset neonatal sepsis. Ampicillin is an accepted alternative, and the choice changes when there is a penicillin allergy.