Is marathon and half marathon training safe during pregnancy?

What does the evidence say about marathon and half marathon training during pregnancy?

1. One review of exercise in pregnant athletes points to a recommendation of avoiding a perceived exertion equivalent to 90% or more of maximum heart rate until further research confirms its safety, an intensity cap that emerged from a case series describing fetal bradycardia when uterine artery blood flow dropped or heart rate reached that threshold, while separately noting VO2max naturally declines in the third trimester on its own. 1
2. The same review does not recommend a fixed reduction formula for competitive athletes, concluding instead that sportswomen should be supported to continue training with individualized adjustments to intensity and activity type, rather than following one standardized cutback plan. 1
3. Heat exposure in any trimester has been associated with preterm birth, stillbirth, and low birthweight, with risk increasing alongside higher temperatures and longer exposure, and as little as one day of high heat may raise risk, which is a specific reason to schedule long training runs and any race itself during the coolest part of the day. 2
4. A systematic review of pregnancy biomechanics found decreased stride length, decreased gait velocity, and increased step width, along with a measurable reduction in peak hip flexion and extension during the second and third trimesters, changes in the mechanics of moving that directly affect running form and pace, not just walking. 3
5. In a small descriptive study of Norwegian elite and recreational athletes who kept exercising at a high volume during pregnancy, elite athletes averaged 11.6 hours of training weekly and recreational athletes 7.0 hours, 76.7% delivered vaginally at term, no babies were born under 2,500 grams, and none of the 50 recreational athletes reported a pregnancy complication, though two of the elite athletes were identified with gestational diabetes and two with high blood pressure. 4

Is marathon and half marathon training safe in each trimester?

• First trimester. If you are already a marathoner, most of your existing routine can continue, though nausea and fatigue often naturally slow pace and mileage on their own. This is not the trimester to begin marathon-distance training for the first time.
• Second trimester. Heat regulation and hydration needs shift as blood volume and body heat both increase, so long runs deserve more attention to fluids and timing now than they did before pregnancy, regardless of how experienced a runner you are.
• Third trimester. Joint laxity, a wider gait, and reduced hip range of motion all measurably change running mechanics by now, which is part of why many experienced pregnant marathoners shift from race-pace training to time-on-feet and perceived effort instead of chasing a specific finish time.