A Popular Class of Antibiotics May Increase Risk of Birth Defects

Headlines about “antibiotics causing birth defects” are the kind of thing that make every pregnant person clutch their prenatal vitamins a little tighter. Recently, a wave of research – and a widely discussed article on Science-Based Medicine – has focused on a very commonly used group of medications: macrolide antibiotics, especially azithromycin (aka the famous Z-Pak). The big question: do these drugs really increase the risk of birth defects, or are we looking at scary statistics without enough context?

In this article, we’ll unpack what the Science-Based Medicine piece actually argued, what large studies have found about macrolides and other antibiotics in pregnancy, and how to balance real risks against the very real danger of untreated infections. We’ll finish with practical tips – and some real-world style experiences – so this doesn’t just feel like reading a pharmacology textbook in disguise.

What Did the Science-Based Medicine Article Actually Say?

The Science-Based Medicine (SBM) article highlighted a large British Medical Journal (BMJ) retrospective cohort study of more than 100,000 pregnancies. In that study, researchers compared women who were prescribed macrolide antibiotics (like azithromycin, erythromycin, and clarithromycin) during pregnancy with women prescribed penicillins, which have a long record of safety in pregnancy.

The key findings from the BMJ study, as summarized by SBM, were:

• Macrolide use in the first trimester was linked to a higher rate of major congenital malformations compared with penicillin.
• The association was strongest for cardiovascular defects (heart malformations).
• When macrolides were used in the second or third trimester, the increased risk was not seen.
• There was no increase in neurodevelopmental disorders (like autism, ADHD, or cerebral palsy).

SBM’s point was not “panic and throw out all macrolides.” Instead, the author emphasized two critical ideas:

1. This was an observational study. It can show association, not prove direct causation.
2. Macrolides are heavily overused. If a drug that’s frequently prescribed for marginal reasons has even a small possible risk in pregnancy, it’s another reason to tighten up prescribing habits.

In other words, the article used the BMJ data as a springboard to talk about antibiotic overuse and the need for better decision-making – not to declare macrolides forbidden forever.

Macrolide Antibiotics 101: Why Are They So Popular?

Macrolide antibiotics include medications like:

• Azithromycin (Zithromax, Z-Pak)
• Erythromycin
• Clarithromycin

They’re popular for very human reasons:

• Once-daily dosing and short courses (that famous 5-day Z-Pak).
• Broad coverage against many respiratory and some sexually transmitted infections.
• Generally well tolerated, with liquid formulations that don’t taste awful.
• A catchy brand image that patients remember and often request by name.

In the U.S., tens of millions of macrolide prescriptions are written every year. But many of these prescriptions are for viral infections – situations where antibiotics do absolutely nothing except increase side effects and resistance. The SBM author rightly pointed out that azithromycin has become a kind of “convenience antibiotic,” which is a problem for everyone, but especially concerning in people who might be pregnant.

Association vs. Causation: Reading the Macrolide Data Carefully

The BMJ macrolide study made headlines, but it’s not the only piece of evidence we have. Some key points from the broader literature:

• Earlier large cohort studies have sometimes found no overall increase in major birth defects with macrolide use in pregnancy, especially when compared with other antibiotics like penicillins and when carefully adjusting for confounding factors.
• Meta-analyses looking specifically at azithromycin have reported results ranging from “no increased risk” to a possible small increase in malformations in some analyses.
• More recent nationwide and multicenter studies continue to find mixed results – some show no significant increase in major birth defects, others see a modest relative increase, particularly in certain heart defects – but the absolute risk remains low.

Why all the inconsistencies? Because these are observational studies, and real life is messy. It’s hard to perfectly separate:

• The effect of the drug, from
• The effect of the underlying infection (which itself can affect the fetus), from
• Other factors like fever, other medications, health conditions, and lifestyle.

So the current science-based position is usually cautious but balanced: macrolides might be associated with a small increase in certain birth defects when taken in early pregnancy, but the data aren’t strong enough to claim a clear, large causal effect. What is clear is that they should not be used casually, especially in the first trimester, when safer alternatives exist.

How Big Is the Risk, Really?

From a patient’s perspective, “55% increased risk” can sound terrifying – until you look at the actual numbers. In the BMJ study that SBM discussed, the authors estimated that:

• Among pregnancies exposed to macrolides in the first trimester, there were roughly 27–28 major malformations per 1,000 births.
• Among pregnancies exposed to penicillin, there were around 17–18 per 1,000.

That’s an increase, but in absolute terms, we’re still talking about a risk that is well under 5% in both groups. Put another way, the study estimated that for every 1,000 pregnant people who receive a macrolide instead of a penicillin in early pregnancy, there may be about four extra babies with a heart defect.

That’s not nothing. But it’s also not “use this drug and your baby will have a birth defect.” It’s a small increase in risk layered on top of a baseline risk that exists even with no medication exposure at all.

This is where nuance matters: for a severe infection where a macrolide is the best or only option, that small possible risk might be acceptable. For a mild illness that doesn’t need antibiotics – or could be treated with a safer drug – it may not be worth it.

What About Other Antibiotics and Birth Defects?

Macrolides aren’t the only antibiotics that have raised eyebrows in pregnancy research. Some important examples:

Sulfonamides and Trimethoprim–Sulfamethoxazole (TMP-SMX)

Several studies have suggested that TMP-SMX, often used for urinary tract infections (UTIs), may be associated with a higher risk of certain congenital malformations when taken in the first trimester. Newer cohort data continue to raise concern that TMP-SMX might carry higher risk compared with beta-lactam antibiotics (like penicillins and cephalosporins).

Professional groups have long cautioned that sulfonamides and TMP-SMX should be used in the first trimester only if other options are not appropriate. More recent research is refining those risk estimates but hasn’t fully erased the concern.

Nitrofurantoin

Nitrofurantoin is another common UTI antibiotic. Older studies suggested possible links with certain birth defects, while more recent data are more reassuring and find no significant overall increase in major malformations compared with beta-lactams when used appropriately.

Guidelines now tend to say that nitrofurantoin is generally acceptable in pregnancy, including early pregnancy, particularly when it’s the best choice for the infection being treated. Again, context and timing matter.

Tetracyclines and Fluoroquinolones

Tetracyclines (like doxycycline) are classic “avoid in pregnancy” antibiotics because of known risks to fetal bones and teeth, especially with repeated or prolonged use.

Fluoroquinolones (like ciprofloxacin and levofloxacin) are more controversial. Some studies have not found a consistent pattern of major birth defects, but there are theoretical concerns about effects on cartilage and tendons, and these drugs already carry significant safety warnings in non-pregnant adults. As a result, they are usually avoided in pregnancy unless absolutely necessary.

The Safer Workhorses: Penicillins and Cephalosporins

The good news: large studies and systematic reviews consistently find that beta-lactam antibiotics – especially most penicillins and cephalosporins – are generally safe in pregnancy and are first-line choices for many infections.

This is exactly why the BMJ macrolide study compared macrolides to penicillins: if you’re going to reach for an antibiotic for a pregnant patient, and you have a safe, effective beta-lactam option, it’s usually the preferred starting point.

Untreated Infection vs. Medication Risk: The Balancing Act

Here’s the part that often gets lost in scary headlines: infections themselves can increase the risk of birth defects and other complications. High fevers, severe systemic illness, untreated UTIs, and sexually transmitted infections can all harm the fetus or lead to preterm birth, low birth weight, or serious maternal illness.

So it’s never as simple as “antibiotics bad, no antibiotics good.” The real question is:

“For this specific pregnant person, with this specific infection, at this specific time in pregnancy, which treatment gives the best balance of benefit and risk?”

That’s why science-based recommendations emphasize:

• Choosing the narrowest effective antibiotic with the best safety profile in pregnancy.
• Being especially cautious with drug choice in the first trimester, when organ development is underway.
• Avoiding unnecessary antibiotics for viral illnesses (no more Z-Pak for a common cold).
• Not withholding antibiotics when they are clearly needed for a serious infection.

In short: the risk of a well-chosen, necessary antibiotic is usually lower than the risk of letting an infection rage unchecked.

Practical Takeaways for Patients and Clinicians

If You’re Pregnant (or Might Be)

• Tell your clinician if you might be pregnant before starting any antibiotic. Early pregnancy is often when risks are most sensitive.
• Ask why a specific antibiotic was chosen. It’s reasonable to ask, “Is there a safer alternative in pregnancy, like a penicillin or cephalosporin?”
• Don’t demand antibiotics for viral illnesses. If your clinician says, “This looks viral,” that’s often good news – you may not need a drug at all.
• Be honest about allergies. Many people who think they’re allergic to penicillin actually aren’t. Penicillin allergy testing can sometimes reopen safer options.
• Never stop an antibiotic abruptly on your own because you read something online. Talk to your prescriber first; stopping halfway can fail to clear the infection and may cause more problems.

If You’re a Clinician

• Start with guideline-supported first-line agents that have strong safety data in pregnancy (e.g., many beta-lactams).
• Use macrolides carefully in the first trimester. Consider whether a beta-lactam would be equally effective and safer.
• Re-evaluate “penicillin allergic” labels. Penicillin allergy delabeling can improve antibiotic stewardship and pregnancy safety.
• Document timing (which trimester), dose, and indication clearly, in case questions about exposure arise later.
• Frame risk in absolute numbers when counseling patients: “Your baseline risk of a major birth defect is about 3–5%; this medication may increase that by a very small amount, and here’s why I still think it’s needed.”

Important note: This article is for educational purposes only and is not a substitute for medical advice. Any decisions about medication use in pregnancy should be made with a qualified healthcare professional who knows your specific situation.

Real-World Experiences: How These Decisions Feel in Practice

Research papers and risk ratios are important, but they don’t fully capture what these choices feel like in real life. To bring the science down to ground level, here are some realistic, composite experiences drawn from common scenarios seen in clinical practice.

Experience 1: The “I Took a Z-Pak Before I Knew I Was Pregnant” Panic

Picture a 30-year-old who took a 5-day azithromycin pack for what she was told was “bronchitis.” Two weeks later, she finds out she’s pregnant – and realizes she was already a few weeks along when she took the antibiotic. Cue the late-night search spiral: “antibiotics birth defects,” “azithromycin pregnancy,” “Z-Pak baby danger.”

In a typical counseling visit, a clinician would walk through:

• Timing: A brief exposure early in pregnancy, already completed, can’t be undone – but it also doesn’t guarantee harm.
• Baseline risk: Even people who take no medications at all have a background risk of major birth defects.
• What the data say: Some studies suggest a small possible increase in risk with macrolides, but the absolute risk remains low, and many large studies are reassuring.
• Next steps: Continue routine prenatal care, possibly with targeted ultrasounds if indicated, and focus on optimizing health going forward.

The emotional shift often happens when patients hear things like, “If I were in your shoes, I would be concerned, but I wouldn’t panic – here’s what we’ll do to watch the baby and support a healthy pregnancy.” Science doesn’t remove the worry, but it can keep it from becoming overwhelming guilt.

Experience 2: Severe Penicillin Allergy and Limited Options

Now imagine a pregnant person with a documented history of anaphylaxis to penicillin who develops a serious pneumonia. A run-of-the-mill “just use amoxicillin” plan is off the table. The team has to weigh macrolides or other broader-spectrum antibiotics that aren’t ideal in pregnancy but may be the best practical choices.

Here, the conversation looks different:

• The infection is dangerous. Severe pneumonia can threaten both maternal and fetal life.
• Alternatives are limited. Some of the “safer” antibiotics can’t be used because of the allergy profile or lack of coverage.
• The risk of not treating the infection clearly outweighs the potential small increase in birth-defect risk from the antibiotic.

In this situation, a macrolide (possibly in combination with another drug) may be entirely appropriate, even in the first trimester. Science-based medicine isn’t about “never use X,” but about using X for the right reasons, at the right time, with full awareness of the tradeoffs.

Experience 3: Rethinking That “Penicillin Allergy” Label

Another common experience is the patient whose chart has “penicillin allergy – rash in childhood” written in big red letters. Because of that one line, clinicians may reflexively reach for macrolides, fluoroquinolones, or TMP-SMX when a simple beta-lactam would otherwise be first-line and safer in pregnancy.

In many clinics and hospitals, allergy services now offer structured “penicillin allergy evaluations,” which might include careful history and sometimes skin testing or supervised oral challenges. When a pregnant patient is found not to be truly allergic, their future treatment options open up. Suddenly, they can safely receive drugs like amoxicillin that have much better pregnancy safety data.

This is a perfect example of how a bit of up-front effort can prevent difficult tradeoffs later. Clearing away inaccurate allergy labels is one of the simplest ways to avoid unnecessary use of higher-risk antibiotics in pregnancy.

Experience 4: Partnering With Patients on Uncertain Data

Finally, there are the countless visits where the science isn’t perfectly settled, and everyone knows it. A pregnant person comes in with a symptomatic UTI or sinus infection, and the clinician explains:

• “We have very good safety data on these antibiotics.”
• “We have mixed or limited data on those antibiotics.”
• “Here’s why I recommend this specific drug, for this long, at this dose.”

When patients are treated like partners – given numbers, context, and space to ask questions – decisions about antibiotics in pregnancy become less about fear and more about informed choice. That is exactly the kind of science-based, patient-centered care the SBM article is advocating.

Conclusion: A Call for Smarter, Not Scarier, Antibiotic Use

The Science-Based Medicine article on macrolides and birth defects doesn’t exist to terrify pregnant people out of taking needed medications. It’s a reminder that:

• Macrolides are widely overused, often for viral illnesses that don’t need antibiotics at all.
• There is a plausible, though small, increased risk of certain birth defects when macrolides are used in early pregnancy, especially compared with safer alternatives like penicillins.
• Most commonly used antibiotics are still quite safe in pregnancy when chosen appropriately.
• Untreated infections carry their own serious risks to both parent and baby.

Science-based medicine isn’t about never taking medication; it’s about taking the right medication, for the right reason, at the right time – with honest counseling about what we know, what we don’t, and how we make the best decision with imperfect information. That’s true for macrolides, and it’s true for every drug we use in pregnancy.