AS and pregnancy: what you need to know before, during, and after

Pregnancy with AS is absolutely possible and has a good prognosis when properly planned. But 'properly planned' is the critical phrase. This is not a situation where you decide in January to start trying in February. The medication picture requires lead time.

Does AS affect fertility?

The good news first: AS itself does not appear to reduce fertility. There is no evidence that having AS makes it harder to conceive.

The caveat: some medications do affect fertility, and they need to be factored in.

Sulfasalazine reduces sperm production in up to 80% of men who take it. The effect is reversible — sperm parameters typically recover within 3-6 months of stopping — but men who are planning to conceive need to switch medications several months before trying, not just stop and hope.

Methotrexate is absolutely contraindicated in pregnancy for both sexes. It is a known teratogen (causes birth defects) and is used medically to terminate ectopic pregnancies. Women must complete a full washout period before conceiving — at least 3 months, often 6. Men are generally also advised to stop methotrexate before their partner tries to conceive, though the male side of the risk is less well-documented.

Planning ahead — the practical timeline

If you are thinking about trying to conceive in the next 1-2 years, the time to have the pregnancy conversation with your rheumatologist is now.

Six months or more before trying: Review all medications with your rheumatologist. If you are on methotrexate, begin the washout process. If you are a man on sulfasalazine, discuss switching.

Three to six months before: Confirm the washout is complete if relevant. Discuss which of your current medications are safe to continue through conception and pregnancy. Begin prenatal folic acid — extra important if you have been on methotrexate.

Once pregnant: Establish a communication pathway between your rheumatologist and your obstetrician. These two teams need to be talking to each other.

What to expect during pregnancy

Many women with AS experience improvement in symptoms during pregnancy — the immune changes of pregnancy, including a shift in the ratio of pro- and anti-inflammatory cytokines, can temporarily dampen AS disease activity. Some women have their best AS months during pregnancy.

Not everyone, though. Peripheral joint symptoms (hips, knees) can worsen as the body produces relaxin to loosen ligaments for delivery. Sacroiliac joint pain in particular can increase as the pelvis changes.

Medication safety in pregnancy

NSAIDs are generally safe in the first and second trimester but should be avoided in the third trimester (risk of premature closing of the ductus arteriosus). Discuss the timing with your rheumatologist.

Biologics: Most TNF inhibitors cross the placenta, particularly in the third trimester. Certolizumab (Cimzia) is unique in that it does not cross the placenta significantly — it lacks the Fc region that mediates placental transfer. For this reason, it is the biologic of choice for pregnancy. Other TNF inhibitors can generally be continued in the first and second trimester with careful management. Secukinumab (IL-17 inhibitor) has less pregnancy data.

Prednisone: Short courses are generally considered acceptable in pregnancy when needed for a flare. Long-term high-dose use carries more risk.

Postpartum flares — the part nobody warns you about

This is the part that blindsides many women: 50-75% of women with AS experience a significant postpartum flare, usually in the weeks and months after delivery.

The immune system, which was suppressed during pregnancy, rebounds sharply after delivery. For women with AS, this rebound can trigger a significant increase in disease activity. It often hits at the same time as severe sleep deprivation, the physical demands of new parenthood, and whatever recovery the birth requires.

This is not unusual. It is not a sign that something has gone wrong. It is a well-documented phenomenon, and the best approach is to plan for it rather than be surprised by it.

Planning means: having your rheumatology team aware and monitoring closely in the first three months postpartum. Having a clear medication plan ready to reinstate if needed. Having practical support in place so that a flare at six weeks postpartum does not also mean having no help with the baby. If you are considering breastfeeding, discussing which medications are compatible — certolizumab has the most data supporting safety during breastfeeding of the biologics.