Azithromycin During Pregnancy & Breastfeeding: Evidence

Being told you need an antibiotic while pregnant or breastfeeding raises a question that rarely comes up at other times: is this safe for my baby, too? Azithromycin sits in an interesting place. It is one of the antibiotics doctors actually recommend in pregnancy for certain infections, yet a widely reported 2020 study raised questions about the macrolide family as a whole. Both things are true, and making sense of them is easier when you see the evidence laid out side by side.

This article uses an “evidence ledger” approach: for each question, we place the reassuring findings and the open questions next to each other, then explain what they mean for real decisions.

Summary: Azithromycin is widely used in pregnancy and is the CDC’s recommended treatment for chlamydia in pregnant people. Large studies have mostly been reassuring, but a 2020 UK study linked macrolides in the first trimester with a small increase in birth defects compared with penicillins, and the UK medicines regulator concluded the data are not strong enough to confirm or rule out a small risk for azithromycin. The usual approach: use azithromycin when it is the best choice for a real infection, prefer a penicillin when that would work equally well, and never leave a significant bacterial infection untreated. During breastfeeding, small amounts pass into milk; it is generally considered acceptable, with monitoring of the baby for diarrhoea, thrush, rash or, rarely in newborns, forceful vomiting.

Why this question is harder than it looks

Pregnant people are usually excluded from drug trials, so safety evidence comes from animal studies, observational studies of people who took the drug, and registries. These studies can show associations but struggle to prove cause. Women prescribed antibiotics are, by definition, ill with something, and infections and fever themselves can affect pregnancy. Researchers try to account for this by comparing one antibiotic against another – for example, macrolides against penicillins – but no method fully removes the uncertainty.

At the same time, untreated bacterial infections carry real, well-documented risks for both parent and baby. Any decision weighs a possible small drug risk against a known infection risk.

When azithromycin is used during pregnancy

Situation	How azithromycin is used	Why it is chosen
Chlamydia	Single 1 g oral dose	CDC-recommended in pregnancy; doxycycline is avoided because of effects on developing teeth and bone
Pneumonia	Often combined with a beta-lactam in hospital	Covers atypical bacteria such as Mycoplasma and Legionella
Whooping cough (pertussis)	Treatment or prevention after close exposure	Preferred macrolide
Preterm prelabour rupture of membranes (PPROM)	Part of a hospital antibiotic regimen	Shown in trials to prolong pregnancy and reduce infection
Non-elective caesarean delivery	Single IV dose added to standard prophylaxis	A large 2016 NEJM trial found fewer infections after surgery
Some other STIs (e.g. chancroid)	Per STI guidelines	Effective option compatible with pregnancy
Penicillin allergy	Alternative for some infections	When a penicillin cannot be used

The evidence ledger: pregnancy

Source	What is reassuring	What remains uncertain
Animal reproduction studies (from the US label)	No evidence of fetal harm at doses well above human exposure	Animal results do not always predict human outcomes
Decades of clinical use and many observational studies	Most studies have not found a clear increase in birth defects with azithromycin specifically	Many studies were small or mixed macrolides together
Fan et al., BMJ 2020 (UK, 104,605 children)	No association found with neurodevelopmental disorders; absolute risk increases were small	First-trimester macrolide use was linked to more major malformations, especially heart defects, vs penicillin; macrolides in any trimester linked to more genital malformations (mainly hypospadias); erythromycin showed the clearest signal
UK Commission on Human Medicines review (after Fan 2020)	Found the data insufficient to confirm a raised miscarriage risk with azithromycin	Also could not rule out a small increased risk of major or cardiac malformations with azithromycin or clarithromycin
Meta-analyses of macrolides in pregnancy	Overall associations with malformations have been weak or inconsistent	Some suggested links to miscarriage or specific defects that need further study
US CDC STI Treatment Guidelines (2021)	Recommend azithromycin as the treatment of choice for chlamydia in pregnancy	Recommendation reflects benefit-risk balance, not proof of zero risk

Putting the 2020 study in perspective

In the Fan study, major malformations were recorded in about 21.6 per 1,000 children whose mothers were prescribed a macrolide during pregnancy, compared with about 17.4 per 1,000 among those prescribed penicillin. The authors estimated that if the link were causal, roughly four additional children per 1,000 might have a heart malformation when a macrolide replaced a penicillin in the first trimester. They were careful to say the study could not prove cause, and that women should not stop antibiotics when needed because untreated infections pose greater risks. Their main suggestion was to use alternatives to macrolides when feasible.

A commentary in clinical literature also noted that some findings, such as associations in later trimesters with defects that form early, are biologically hard to explain, which is one reason the results are treated as a signal to watch rather than a settled conclusion.

Trimester-by-trimester view

Stage	What is developing	Practical considerations
Before you know you are pregnant (first 2–4 weeks after conception)	Very early cell division	Exposures this early often have an “all or nothing” effect; most pregnancies continue normally. Tell your clinician about any exposure
First trimester (to 13 weeks)	Organs form, including heart	The period of most concern in the 2020 study; clinicians may prefer a penicillin when it would work equally well
Second trimester (14–27 weeks)	Growth and maturation	Azithromycin used when indicated; infection control is the priority
Third trimester (28 weeks to birth)	Growth, lung maturation	Used for chlamydia, pneumonia, PPROM; chlamydia treatment protects the newborn from eye and lung infection
Labour and delivery	Birth	Single dose added for some caesareans; very close to birth, the baby may be exposed briefly

The other side of the scale: untreated infection

It is easy to focus only on the medicine. These are some of the documented risks of leaving infections untreated in pregnancy:

Chlamydia: linked to preterm birth and can pass to the baby during delivery, causing conjunctivitis and pneumonia in the newborn.
Pneumonia: pregnancy changes lung capacity and immunity, so pneumonia can become severe quickly; low oxygen levels can affect the baby.
Whooping cough: very dangerous for newborns, who may catch it from family members.
PPROM-related infection: can lead to chorioamnionitis and serious newborn infection.

This is why obstetric and infectious disease guidelines continue to recommend azithromycin in specific pregnancy situations.

Azithromycin vs other antibiotic options in pregnancy

Antibiotic	Pregnancy profile in brief	Typical role
Amoxicillin / penicillins	Large body of reassuring data	First choice for many infections; alternative chlamydia regimen in pregnancy (7-day course); see amoxicillin in pregnancy
Cephalosporins	Generally considered safe	Many respiratory, urinary and surgical uses
Azithromycin	Widely used; small uncertain signal for macrolides as a group	Chlamydia, atypical pneumonia, pertussis, PPROM, caesarean prophylaxis add-on
Erythromycin	Clearest malformation signal in the 2020 study; more stomach upset	Less commonly chosen now
Clarithromycin	Less reassuring data; some guidance advises avoiding unless no alternative	Rarely first choice
Doxycycline and other tetracyclines	Generally avoided after early pregnancy because of effects on teeth and bones	Not used for chlamydia in pregnancy
Fluoroquinolones	Usually avoided unless no alternative	Specialist decisions only

If you took azithromycin before you knew you were pregnant

This happens often, and in the vast majority of cases the pregnancy continues normally. Here is what to do:

Don’t panic. The absolute risk of any problem from a short course is low; most babies exposed early are born healthy.
Note the details: the date of your last menstrual period, when you took the azithromycin, the dose and number of days, and why it was prescribed.
Tell your obstetrician or midwife at your first appointment. They will consider it alongside your routine anomaly scan.
Finish any treatment still needed only after discussing with your doctor, especially if it was for an STI, since partner treatment and retesting may matter.
Consider a specialist information service. In the US and Canada, MotherToBaby offers free expert advice by phone, text or chat. In the UK, the UK Teratology Information Service (UKTIS) publishes “Best Use of Medicines in Pregnancy” leaflets.

Chlamydia in pregnancy: a closer look

Because chlamydia is a common reason for azithromycin in pregnancy, it deserves its own checklist:

Take the single dose as directed, ideally observed in the clinic.
Avoid sex for seven days after treatment and until your partner(s) have been treated.
Have a test of cure about four weeks after treatment – this is recommended specifically in pregnancy.
Get retested about three months later, and, depending on your risk, again in the third trimester.
Ask about testing for other infections, such as gonorrhoea, syphilis and HIV.

Our article azithromycin for chlamydia covers partner treatment and retesting in detail.

Breastfeeding: the evidence ledger

Question	Reassuring findings	Points to watch
Does it get into milk?	Yes, but in small amounts; babies are often given azithromycin directly in much larger doses for infections	Infant levels have not been well measured
Effects on the baby’s gut?	Most babies show no problems	Possible loose stools, diaper rash or oral thrush from changes in gut flora
Pyloric stenosis risk?	Two meta-analyses did not find a relationship between maternal macrolide use while breastfeeding and infant pyloric stenosis	Earlier observational studies, including a Danish database study, suggested higher risk when mothers took macrolides in the first two weeks after birth; evidence is unconfirmed
Comparison with amoxicillin	A study found adverse reaction rates in macrolide-exposed nursing infants similar to amoxicillin-exposed infants	Few of those infants were exposed specifically to azithromycin
Overall verdict	LactMed and most lactation resources consider azithromycin acceptable during breastfeeding	Extra watchfulness in newborns under about two weeks

What to watch for in a breastfed baby

Diarrhoea or unusually loose, frequent stools
Diaper rash or white patches in the mouth (thrush)
Skin rash
Forceful, projectile vomiting after feeds, constant hunger or poor weight gain – contact the baby’s doctor promptly, especially in the first weeks of life

Do not stop breastfeeding without talking to your doctor or a lactation consultant; for most short courses, continuing to nurse is recommended.

Practical tips during a course

Take azithromycin with food if it upsets your stomach – nausea in pregnancy can make antibiotic side effects feel worse.
Stay well hydrated, especially with vomiting or diarrhoea.
Separate doses from aluminium- or magnesium-containing antacids, which are commonly used for pregnancy heartburn. See azithromycin drug interactions.
Avoid alcohol, as recommended throughout pregnancy.
Tell your doctor about any heart rhythm issues or fainting, since azithromycin can affect QT interval. See azithromycin side effects.
Report worsening symptoms promptly – fever, breathlessness, abdominal pain, leaking fluid or reduced baby movements need same-day review.

Questions to ask your obstetrician or midwife

What infection do you think I have, and how could it affect the pregnancy if untreated?
Is azithromycin the best option, or would a penicillin work equally well for me?
Does the stage of my pregnancy change your choice?
Do I need any follow-up tests to confirm the infection has cleared?
Does my partner need testing or treatment?
If I’m breastfeeding, what should I watch for in my baby?
Are any of my pregnancy medicines likely to interact?

Four common pregnancy scenarios

Scenario 1: Chlamydia found at the 10-week booking visit

Routine screening finds chlamydia. The clinician recommends a single 1 g azithromycin dose, ideally taken in the clinic. The patient asks about the first-trimester study. The clinician explains that untreated chlamydia raises the risk of preterm birth and newborn infection, that doxycycline is avoided in pregnancy, and that amoxicillin is an alternative if she prefers to avoid a macrolide – though it needs three doses a day for a week. Together they decide; test of cure follows in four weeks, and her partner is treated.

Scenario 2: Pneumonia at 30 weeks

A pregnant patient has fever, cough and low oxygen levels. She is admitted and given a beta-lactam antibiotic plus azithromycin to cover atypical bacteria. Here, treating promptly is clearly the priority: pneumonia in late pregnancy can become serious quickly for both parent and baby.

Scenario 3: Whooping cough exposure in the household

An older child at home has confirmed pertussis. Because newborns are especially vulnerable, a clinician may offer azithromycin to the pregnant parent as post-exposure prevention, particularly close to the due date. Vaccination during each pregnancy (Tdap in the US, pertussis vaccine in the UK) is also strongly recommended to protect the baby after birth.

Scenario 4: Traveller’s diarrhoea while pregnant

A pregnant traveller develops severe diarrhoea with fever. Fluoroquinolones are usually avoided in pregnancy, so azithromycin may be chosen if an antibiotic is needed. Oral rehydration remains the most important treatment, and dehydration in pregnancy warrants medical review.

Trying to conceive and partners

If you’re planning pregnancy: finishing a short azithromycin course is not a reason to delay trying to conceive, but treating an STI fully – including partners – is important, because untreated chlamydia can affect fertility.
Male partners: there is no evidence that a man taking azithromycin around the time of conception harms a pregnancy. Treating his infection protects his partner.
Fertility treatment: some IVF clinics screen for chlamydia and treat it before procedures. Follow the clinic’s protocol.

Myths and facts

Myth	Fact
“All antibiotics are dangerous in pregnancy.”	Many are used routinely; the choice depends on the drug, the infection and the stage of pregnancy.
“It’s safer to wait until after the baby is born.”	Untreated infections like chlamydia or pneumonia can harm the pregnancy and newborn.
“The 2020 study proved azithromycin causes heart defects.”	It found an association for macrolides as a group, mainly erythromycin; it could not prove cause, and regulators judged the azithromycin data inconclusive.
“If I took it before knowing I was pregnant, I should consider ending the pregnancy.”	Short exposures rarely change outcomes; talk to your clinician or a teratology service before making any decision.
“Breastfeeding mothers must stop nursing during antibiotics.”	For azithromycin, continuing to breastfeed is generally recommended, with monitoring of the baby.

Pregnancy and heart rhythm

Pregnancy itself causes changes in heart rate and blood volume. Most pregnant people have no QT concerns, but tell your clinician if you have a history of fainting, palpitations, a known long QT, or take medicines such as ondansetron for severe nausea – which also affects QT. Hyperemesis gravidarum with repeated vomiting can lower potassium and magnesium, which matters when azithromycin is added. See azithromycin drug interactions.

A note on anxiety

Medicine worries in pregnancy are common and understandable. Reading headlines about “birth defect” studies can be distressing, especially after a positive STI test, which can carry its own emotional weight. If worry is affecting your sleep or daily life, tell your midwife or doctor; support is part of good antenatal care. Our guide on how to calm an anxiety attack has practical techniques.

How doctors weigh the decision

When an infection needs treatment during pregnancy, clinicians usually work through a few questions in order:

Is the infection bacterial and does it need treatment now? Viral illnesses don’t need antibiotics; some bacterial infections can’t safely wait.
Which antibiotics reliably treat it? For chlamydia in pregnancy, the CDC lists azithromycin as recommended and amoxicillin as an alternative. For atypical pneumonia, a macrolide is often needed.
Among effective options, which has the most reassuring pregnancy data? Penicillins and cephalosporins usually rank highest; azithromycin is widely used; tetracyclines and fluoroquinolones are generally avoided.
Will the patient be able to complete the course? A single dose may outweigh a slightly more reassuring three-times-a-day regimen if adherence is a concern.
Are there personal factors? Allergies, heart rhythm issues, other medicines and stage of pregnancy all matter.

Sharing your own priorities – for example, concern about first-trimester exposure, or worry about remembering doses – helps your clinician tailor the choice to you.

Frequently asked questions

Is azithromycin safe in the first trimester?

It is used in the first trimester when needed, such as for chlamydia. A 2020 UK study found a small association between first-trimester macrolide use and heart malformations compared with penicillin, and UK regulators could not rule out a small risk for azithromycin. Many clinicians prefer a penicillin when it would be equally effective, but treating a genuine infection remains important.

Can azithromycin cause miscarriage?

Some studies have suggested a link between macrolides and miscarriage, but the UK review found the data insufficient to confirm an increased risk with azithromycin. Infections themselves can raise miscarriage risk, which complicates these studies.

Why do doctors give azithromycin for chlamydia in pregnancy instead of doxycycline?

Doxycycline, the preferred treatment outside pregnancy, is generally avoided because tetracyclines can affect a baby’s developing teeth and bones. Azithromycin is effective as a single dose and is the CDC’s recommended option in pregnancy.

Can I breastfeed while taking azithromycin?

In most cases, yes. Small amounts reach breast milk. Watch your baby for diarrhoea, thrush or rash, and contact a doctor about projectile vomiting, especially in a newborn.

Do I need to pump and dump?

Generally no. Discarding milk is not routinely recommended for azithromycin. Ask your doctor if your baby is premature or unwell.

My baby was given azithromycin directly. Is that the same risk?

Infants receive azithromycin for conditions like whooping cough. In very young infants, doctors monitor for pyloric stenosis. The dose a baby gets directly is far higher than from breast milk.

Can azithromycin harm my baby later in pregnancy?

Studies have not shown clear harm from azithromycin in the second or third trimester, and it is used for pneumonia, PPROM and chlamydia at these stages. The 2020 study reported a possible link between macrolides in any trimester and hypospadias, a finding experts have questioned as biologically less plausible.

Is the Z-Pak safe during pregnancy?

The five-day course may be used when azithromycin is the right antibiotic for the infection. Your clinician will weigh whether a penicillin would work equally well.

Sources

Medical disclaimer: This article is for general information and does not replace individual advice from your obstetrician, midwife, doctor or pharmacist.