Erythromycin is generally considered safe during pregnancy and has been prescribed to pregnant women for decades, but the picture is not as simple as a blanket yes. Large population studies have found no overall increase in birth defects when erythromycin is used in the first trimester, and no link to miscarriage. However, a few specific concerns have surfaced over the years, including a disputed signal around heart defects, a liver toxicity issue tied to one particular formulation, and practical problems with side effects and drug absorption that can make it a less-than-ideal choice compared to newer alternatives.
Overall Risk of Birth Defects
The broadest reassurance comes from several large register-based studies that tracked tens of thousands of pregnancies. A Danish nationwide cohort study comparing macrolide use (including erythromycin) to penicillin use found no meaningful difference in birth defects: about 35 per 1,000 births in the macrolide group versus 37 per 1,000 in the penicillin group. When erythromycin was analyzed individually, there was no significant link to any major birth defect compared to penicillin.1BMJ. Association between use of macrolides in pregnancy and risk of major birth defects: nationwide, register based cohort study A Norwegian population-based study reached a similar conclusion, finding that the risk for any specific malformation was not increased with erythromycin or other macrolides compared to no antibiotic use in the first trimester.2PubMed Central. Pregnancy outcome after gestational exposure to erythromycin – a population-based register study from Norway
A large French cohort study added a wrinkle. When macrolides as a class were compared to amoxicillin, there was no overall increase in birth defects. But the study did detect a small increase in the risk of two rare defects: spina bifida and syndactyly (fused fingers or toes).3PLOS Medicine. First-trimester exposure to macrolides and risk of major congenital malformations compared with amoxicillin: A French nationwide cohort study These findings involved the entire macrolide class rather than erythromycin specifically, and the absolute risk remained very low. Still, they are worth knowing about, especially because some of the earlier and larger studies did not detect the same signal.
The Cardiovascular Defect Question
One of the longest-running debates around erythromycin in pregnancy involves heart defects. A Swedish study from 2005 reported a roughly doubled risk of cardiovascular malformations after first-trimester erythromycin exposure compared to no antibiotic use.4PubMed. Is erythromycin therapy teratogenic in humans? That finding alarmed prescribers and was widely cited for years. A European case-control study added some support, finding a roughly three-to-fourfold increased risk of one specific heart defect, atrioventricular septal defect, in pregnancies exposed to macrolides including erythromycin.5PubMed. Macrolide and lincosamide antibiotic exposure in the first trimester of pregnancy and risk of congenital anomaly: A European case-control study
The problem is that the larger, more recent cohort studies have not confirmed this. The Danish study mentioned above, which directly compared macrolide users to penicillin users, found no increased risk of cardiovascular defects for any individual macrolide, including erythromycin.1BMJ. Association between use of macrolides in pregnancy and risk of major birth defects: nationwide, register based cohort study The disagreement likely comes down to study design. Smaller case-control studies are more vulnerable to bias from confounding factors, such as the underlying infection being treated, whereas large cohort studies with an active comparator (penicillin instead of “no antibiotic”) do a better job of separating the drug’s effect from the illness itself. The weight of evidence currently tips away from a true cardiovascular risk, but researchers have not entirely closed the book.
Pyloric Stenosis and Prenatal Exposure
You may have heard that erythromycin causes pyloric stenosis, a condition where the muscle at the exit of a baby’s stomach thickens and blocks food from passing through. That concern is well-established for erythromycin given directly to newborns in the first two weeks of life. But prenatal exposure is a different question.
Two studies specifically examined whether erythromycin taken by a pregnant woman increases pyloric stenosis risk in her baby. One found no association at any point during pregnancy.6American Journal of Obstetrics and Gynecology. Erythromycin use during pregnancy in relation to pyloric stenosis The other found no significant link whether erythromycin was used after 32 weeks or at any time during pregnancy.7Obstetrics & Gynecology. Prenatal prescription of macrolide antibiotics and infantile hypertrophic pyloric stenosis A third study raised the possibility that maternal macrolide use within 10 weeks of delivery might carry some risk, but described its own data as inconclusive.8PubMed. Maternal and infant use of erythromycin and other macrolide antibiotics as risk factors for infantile hypertrophic pyloric stenosis The consensus is that prenatal erythromycin does not meaningfully raise pyloric stenosis risk in the baby, though the drug should not be given directly to very young infants.
Miscarriage Risk
Two studies looked specifically at whether erythromycin in early pregnancy raises the chance of miscarriage. A Danish study found no increased hazard at all, with the risk essentially identical to that of unexposed pregnancies.9PLOS ONE. Clarithromycin in Early Pregnancy and the Risk of Miscarriage and Malformation: A Register Based Nationwide Cohort Study A Canadian study actually found a slightly reduced risk of spontaneous abortion with erythromycin, though the confidence interval just crossed the line of statistical significance.10CMAJ. Use of antibiotics during pregnancy and risk of spontaneous abortion Neither study found cause for concern. This is in contrast to some other antibiotics (such as metronidazole and fluoroquinolones) that have drawn more scrutiny for miscarriage risk.
The Estolate Formulation and Liver Damage
Erythromycin comes in several salt forms: erythromycin base, stearate, ethylsuccinate, and estolate. Most of them behave similarly in pregnancy. The exception is erythromycin estolate, which is linked to liver problems in pregnant women.
A controlled trial from the 1970s found that about 10% of pregnant women who received erythromycin estolate developed abnormally elevated liver enzymes, compared to under 2.5% in the placebo group. The researchers concluded that the estolate form may be inadvisable in pregnancy.11PubMed Central. Hepatotoxicity of erythromycin estolate during pregnancy Case reports have also described full-blown cholestatic jaundice, with symptoms including itching, nausea, and yellow skin lasting weeks to months.12JAMA. Cholestatic Jaundice in Patients Treated with Erythromycin Estolate This is a hypersensitivity reaction rather than a dose-dependent toxicity, meaning it can happen at standard doses and is hard to predict.
The practical takeaway is straightforward: if you are prescribed erythromycin during pregnancy, make sure it is not the estolate formulation. Most prescribers know this, and in many countries the estolate form is rarely used at all. But it is worth checking the label or asking your pharmacist, especially if you are filling a prescription from a different healthcare system.
How Much Erythromycin Reaches the Fetus
One reason erythromycin has been considered relatively safe is that the placenta does not allow very much of it through. In a perfusion model that mimicked maternal and fetal blood flow, erythromycin’s mean transfer rate was only about 3%, meaning that the fetus is exposed to a tiny fraction of the mother’s blood levels.13PubMed. The transplacental transfer of the macrolide antibiotics erythromycin, roxithromycin and azithromycin A separate experiment found that while the concentration in the maternal circuit dropped by about a third, the fetal-side concentration did increase by roughly two-thirds from its baseline, suggesting some drug does get across.14PubMed Central. Placental transport of Erythromycin and its effect on placental inflammatory factors The low transfer rate is reassuring for fetal safety but has a clinical downside: it means erythromycin may not be the best choice when the goal is to treat a fetal infection or to clear bacteria from the amniotic fluid, because so little drug actually gets to where it would need to act.
Gastrointestinal Side Effects and Compliance
Erythromycin’s biggest practical problem in pregnancy is that many women cannot tolerate it. The drug is a strong stimulant of gut motility, and nausea, vomiting, and abdominal cramps are common. In one trial comparing erythromycin to azithromycin for chlamydia treatment in pregnancy, about two-thirds of women on erythromycin reported gastrointestinal side effects, compared to fewer than one in five on azithromycin.15PubMed Central. Randomized Clinical Trial of Azithromycin vs. Erythromycin for the Treatment of Chlamydia Cervicitis in Pregnancy In that same study, the side effects were directly tied to treatment failure, because women who felt sick simply stopped taking the pills.
Another study using a lower dose of erythromycin (the standard 250 mg four times daily regimen) reported an intolerance rate of about 3%, though this was defined as women who stopped the drug entirely rather than all who had symptoms.16PubMed. Experience with the routine use of erythromycin for chlamydial infections in pregnancy The gap between “had symptoms” and “stopped treatment” can be wide. Many women push through mild nausea, especially if they understand the medical reason for finishing the course. But in a population already dealing with pregnancy-related nausea, adding a drug known for stomach upset is not ideal.
Absorption Problems in Late Pregnancy
Even if you manage to keep erythromycin down, your body may not absorb it well. A small study of women in the third trimester found that absorption was delayed and blood levels were lower than what is typically seen outside of pregnancy. In two of the women studied, erythromycin was undetectable in the blood at any point over four hours, and those two women also had the worst gastrointestinal symptoms.17PubMed. Serum erythromycin levels in pregnancy A separate study noted that individual variability in erythromycin blood levels was greater in pregnant women than what has been reported in non-pregnant people, though multiple doses eventually produced adequate levels in most subjects.18PubMed. Erythromycin and clindamycin absorption and elimination in pregnant women
The clinical worry here is that if gastrointestinal distress predicts poor absorption, the drug may fail for two reasons at once: the patient feels terrible and the antibiotic is not reaching effective levels. This is one reason why clinicians increasingly prefer azithromycin, which requires fewer doses and achieves high tissue concentrations more reliably.
Erythromycin for Preterm Premature Rupture of Membranes
One of the most important clinical uses of erythromycin in pregnancy is for preterm premature rupture of membranes (PPROM), when the amniotic sac breaks before 37 weeks. Antibiotics are given to delay delivery and reduce the chance of infection. Erythromycin has been a cornerstone of PPROM treatment protocols for years. A Canadian clinical guideline recommends either a regimen of intravenous ampicillin plus erythromycin followed by oral amoxicillin plus erythromycin, or oral erythromycin alone for 10 days.19PubMed. Antibiotic therapy in preterm premature rupture of the membranes
The evidence for erythromycin in this setting is real but modest. A randomized trial of 220 women with PPROM found that erythromycin did extend the time between membrane rupture and delivery, particularly in women who went on to develop infection. However, it did not reduce the overall rate of maternal or neonatal infection, and did not lower newborn illness or death.20PubMed. Erythromycin therapy in preterm premature rupture of the membranes: a prospective, randomized trial of 220 patients In other words, it buys time but does not prevent the complications that follow. More recent data have explored whether azithromycin performs just as well in this role. A comparison found no difference in how long delivery was delayed between azithromycin and erythromycin regimens, and delivery complications were similar across groups.21American Journal of Obstetrics & Gynecology. Azithromycin vs erythromycin for the management of preterm premature rupture of membranes
Erythromycin for Chlamydia in Pregnancy
Before azithromycin became widely available, erythromycin was the standard treatment for chlamydia during pregnancy. It remains effective, with cure rates around 96% in trials that directly compared it to amoxicillin and clindamycin.22PubMed Central. Randomized prospective study comparing erythromycin, amoxicillin, and clindamycin for the treatment of chlamydia trachomatis in pregnancy A Cochrane review found that azithromycin and erythromycin probably have similar cure rates for chlamydia in pregnancy, with results slightly favoring azithromycin.23Cochrane Database of Systematic Reviews. Interventions for treating genital Chlamydia trachomatis infection in pregnancy
Where azithromycin clearly wins is on side effects and compliance. A meta-analysis of randomized trials found that azithromycin and erythromycin had similar effectiveness, but azithromycin caused dramatically fewer gastrointestinal side effects, fewer dropouts, and much better compliance. The compliance advantage was enormous, with patients on azithromycin roughly 24 times more likely to complete treatment.24PubMed. Single-dose azithromycin versus erythromycin or amoxicillin for Chlamydia trachomatis infection during pregnancy: a meta-analysis of randomised controlled trials When a drug works in theory but patients cannot finish taking it, real-world effectiveness drops. This is why most current guidelines now favor azithromycin (a single dose) over erythromycin (seven to 14 days of multiple daily doses) for chlamydia in pregnancy.
Drug Interactions Worth Knowing About
Erythromycin inhibits a liver enzyme called CYP3A4, which the body uses to process many common medications. Even a standard 250 mg dose reduces the activity of this enzyme by roughly 40%, and at higher doses the inhibition is more pronounced.25PubMed Central. Systemic exposure of topical erythromycin in comparison to oral administration and the effect on cytochrome P450 3A4 activity In pregnancy, this matters because women may be taking other drugs that rely on CYP3A4 for clearance. Anti-nausea medications, certain blood pressure drugs, some anti-seizure medications, and even some over-the-counter antihistamines can be affected. If you are prescribed erythromycin alongside other medications, make sure your prescriber reviews potential interactions, especially for drugs with a narrow therapeutic window where a small change in blood level matters.
Rising Resistance and the Shift Away from Erythromycin
Beyond tolerability and absorption, there is another reason erythromycin is losing ground as a go-to antibiotic in pregnancy: bacteria are increasingly resistant to it. A study at one U.S. hospital found that over half of Group B Streptococcus cultures were resistant to erythromycin, a rate far higher than older national estimates had suggested.26PubMed Central. High rates of perinatal group B Streptococcus clindamycin and erythromycin resistance in an upstate New York hospital GBS is one of the most important bacteria to manage during pregnancy because it can cause serious infections in newborns. When resistance rates climb this high, prescribing erythromycin as a backup for penicillin-allergic women becomes unreliable without first testing the specific bacteria for susceptibility.
This pattern of rising resistance extends beyond GBS. Erythromycin resistance among common respiratory and skin bacteria has also been climbing for years. The practical effect is that even when erythromycin is safe for the mother and fetus, it may not work against the infection it is supposed to treat. Your prescriber should be checking local resistance patterns and, when possible, ordering susceptibility testing before choosing erythromycin over alternatives.
The Effect on the Newborn Microbiome
Any antibiotic taken during pregnancy can alter the bacterial communities that a mother passes to her baby at birth. Epidemiological research has linked prenatal antibiotic exposure broadly to shifts in the infant gut microbiome, with potential downstream effects on immune development and disease risk in childhood. This is not unique to erythromycin; it applies to antibiotics as a class. The evidence connecting any specific antibiotic to specific long-term outcomes in children remains preliminary, and the risk of leaving a genuine infection untreated almost always outweighs hypothetical microbiome concerns. Still, it is one more reason that antibiotics in pregnancy should be used when truly needed, not as a precaution against vague risk.
One large trial, the MRC ORACLE study, enrolled thousands of women with PPROM or spontaneous preterm labor and randomized them to erythromycin, co-amoxiclav, both, or placebo. A follow-up study of the children born in that trial was designed to look for long-term effects on development and health.27PubMed Central. MRC ORACLE Children Study. Long term outcomes following prescription of antibiotics to pregnant women with either spontaneous preterm labour or preterm rupture of the membranes The very existence of that follow-up study reflects the fact that the medical community takes the question of long-term effects seriously, even when short-term safety data are reassuring.