Starting low-dose aspirin at 6 or at 13 weeks made no difference to its effect on preterm birth

Design

Post hoc analysis of a multicentre placebo-controlled randomised trial

Population

11,908 women randomised at 6–13 weeks in Africa, Asia and Latin America

Primary outcome

Preterm delivery, by week of starting and by adherence

Effect

Per week later: RR 0.97 (0.93–1.02); per 5% adherence: RR 1.01 (0.99–1.04)

This post hoc analysis of the ASPIRIN trial, published in April, used data from 11,908 nulliparous women randomised at 6–13 weeks in low-resource settings in Africa, Asia and Latin America. Median gestation at starting was 10.1 weeks, and 85.5% of women took more than 90% of their tablets by pill count.

For each week later that aspirin was started, the treatment effect on preterm birth did not change (RR 0.97, 95% CI 0.93–1.02). The same held for preterm birth before 34 weeks (0.98, 0.89–1.07) and perinatal mortality (1.04, 0.96–1.12). Each 5% increase in adherence did not change the effect either (RR for preterm birth 1.01, 0.99–1.04), and there was no difference by region.

For practice, a woman booking at 12 or 13 weeks has not missed the window the trial tested. Adherence was very high overall, so the analysis says little about women who take aspirin only occasionally.

• Offer low-dose aspirin at booking even when booking is late in the first trimester
• Do not delay starting to wait for a dating scan if the indication is clear
• Ask about adherence at each visit — the trial's high adherence may not match your clinic
• Note that this trial studied preterm birth in nulliparous women, not only those at high pre-eclampsia risk

Why it matters

Late first-trimester booking is common, and this removes a reason to think aspirin is no longer worth starting.

Don't overread it

This was a post hoc analysis with very high adherence, so it cannot show that taking aspirin irregularly works as well.

The statistics, in plain English

These ratios measure how much the treatment effect changes per week or per 5% adherence. A value of 1.0 means no change, and every interval here includes 1.0. Because 85% of women had over 90% adherence, there were few poorly adherent women to compare, which limits what the adherence result can show.

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