Table 1 Characteristics of included studies

Sibai 1993

We enrolled nulliparous women seeking prenatal care who were 13 to 25 weeks pregnant if their blood pressure was below 135/85 mmHg and they had no proteinuria on testing with a dipstick (sensitivity, less than 30 mg per deciliter if the dipstick result is zero or trace). Women with chronic hypertension, renal disease, diabetes mellitus, and other medical illnesses were excluded.

3135

Compliance with treatment was assessed by asking about tablet intake and by counting tablets. The estimated percentage of the prescribed tablets taken averaged 83% in the aspirin group and 84% in the placebo group. 72% of the women in the aspirin group and 74% of the women in the placebo group took 80% or more of the tablets prescribed between the start of the study and delivery.

60 mg daily

Placebo

From recruitment until the onset of labor

Hauth 1993

Nulliparous, healthy, and with a singleton gestation at between 20 and 22 weeks’ gestation.

604

We assessed patient compliance by a capsule count at each 2-week visit. The mean compliance index, with a maximum of 100, was 94.5 (SD ± 5.4) in the aspirin group and 94.4 (SD ± 7.53) in the placebo group (p = 0.43, not significant).

60 mg daily

Placebo

From 24 weeks until delivery.

Davies 1995

Healthy, nulliparous women with a high hemoglobin concentration in the second trimester.

118

At each antenatal visit women were asked about the compliance. They described that compliance was excellent but without specific data.

75 mg daily

Placebo

From 18 weeks until delivery.

Golding 1998

Nulliparous women enrolled between 12 and 32 weeks of gestation.

6275

Compliance was measured by counting the remaining tablets and was recorded only if bottles were available at each visit and remaining pills were consistent with expectation. The authors considered that compliance in this study was poor.

60 mg daily

Placebo

From recruitment until delivery

Rotchell 1998

All women attending antenatal clinics between 12 and 32 weeks of gestation were eligible, if without specific contraindications to aspirin and unlikely to deliver immediately.

3647

Compliance was measured by calendar sheets. Each treatment pack contained seven one-month calendar sheets. 55% of women reported taking tablets for more than 80% of the time.

75 mg daily

Placebo

From recruitment until delivery

Subtil 2003

Women were eligible if they were nulliparous (no previous delivery at or after 22 weeks), at a gestational age between 14 and 20.6 weeks, planned to continue prenatal care and give birth in the participating facility and provided written informed consent.

3274

Two methods were used to assess the compliance: counting remaining packets and salicyluric acid assays during the seventh month of pregnancy in some patients. The last day of treatment was known for 88.7% of the patients. Of them, 83.7% reported treatment compliance of at least 80%.

100 mg daily

Placebo

From 14–20 weeks untill 34 weeks.

Hoffman 2020

Nulliparous women between 14 and 40 years of age with an ultrasound confirming gestational age and singleton viable pregnancy in multi-country. Women were excluded if had any of the following by medical history: allergy or contraindication to aspirin, previously taken aspirin therapy for more than 7 days during this pregnancy, multiple gestations, history of more than two first trimester losses, or medical conditions for which low-dose aspirin therapy is currently indicated (e.g. diabetes and hypertension).

11,976

A pre-packaged 2-week medication allotment was exchanged every 2 weeks by study personnel to assess compliance. Overall adherence to medication or placebo defined as taking ≥ 90% of the prescribed medications was high (MITT population overall 84.9%: aspirin 85.3% vs. placebo 84.4%).

81 mg daily

Placebo

Initiated between 6 weeks and 0 days and 13 weeks and 6 days until 36 weeks and 0 days of pregnancy