Threats to reliability risk erroneous conclusions: a survey of prospective registration and sample sizes of randomised trials in women's health

Incidences of obstetric outcomes and sample size calculations: A Danish national registry study based on all deliveries from 2008 to 2015

Introduction

In high‐income countries the majority of pregnancies have a good outcome, and many adverse obstetric outcomes rarely occur. This makes demonstrating clinically relevant and statistically significant effects of new interventions a challenge. The objective of the study was to report incidences of important obstetric outcomes and to calculate sample sizes for tentative studies.

Material and methods

The study was a registry‐based study. Data were retrieved from the Danish Medical Birth Registry and included all deliveries in Denmark from 2008 to 2015. The total population included 465 919 deliveries. The study population comprised intended vaginal deliveries with a single fetus in cephalic presentation at term (n = 381 567). Incidences were reported for 20 outcomes considering the relevance for the patients and the severity of the outcomes. We calculated the sample sizes required in tentative obstetric studies to detect risk reductions of 25 and 50%, for tests at the 5% level, using a power of 80 and 90%. For the randomized controlled trials we calculated the sample size required for comparing two proportions with equal‐sized groups. For the cohort study we calculated the sample size also required for two proportions but with unequal sized groups. Outcome measures for sample size calculation were neonatal mortality, Apgar score <7 at 5 minutes and emergency cesarean section.

Results

The incidence of neonatal mortality, Apgar score <7 at 5 minutes and emergency cesarean section was 0.05, 0.58 and 10.5%, respectively. Using neonatal mortality as the outcome in a tentative randomized controlled trial with an expected risk reduction of 50% and power of 80%, our calculation showed a sample size of 195 036 deliveries. Using Apgar score <7 at 5 minutes or emergency cesarean section as the outcome, 16 254 and 818 deliveries, respectively, were required. In tentative cohort studies, the required sample sizes were larger due to the unequal proportion of exposed/non‐exposed women.

Conclusions

Most adverse obstetric outcomes occur rarely; thus, very large sample sizes are required to achieve adequate statistical power in randomized controlled trials. Multicenter studies, international collaborations or alternative study designs to randomized controlled trials could be considered.

Feasibility and acceptability of expressive writing with postpartum women: a randomised controlled trial

Background

Pregnancy, birth and adjusting to a new baby is a potentially stressful time that can negatively affect women’s mental and physical health. Expressive writing, where people write about a stressful event for at least 15 min on three consecutive days, has been associated with improved health in some groups but it is not clear whether it is feasible and acceptable for use with postpartum women. This study therefore examined the feasibility and acceptability of expressive writing for postpartum women as part of a randomised controlled trial (RCT).

Methods

The Health After Birth Trial (HABiT) was an RCT evaluating expressive writing for postpartum women which included measures of feasibility and acceptability. At 6 to 12 weeks after birth 854 women were randomised to expressive writing, a control writing task or normal care, and outcome measures of health were measured at baseline, one month later and six months later. Feasibility was measured by recruitment, attrition, and adherence to the intervention. Quantitative and qualitative measures of acceptability of the materials and the task were completed six months after the intervention.

Results

Recruitment was low (10.7% of those invited to participate) and the recruited sample was from a restricted sociodemographic range. Attrition was high, increased as the study progressed (35.8% at baseline, 57.5% at one month, and 68.1% at six months) and was higher in the writing groups than in the normal care group. Women complied with instructions to write expressively or not, but adherence to the instruction to write for 15 min per day for three days was low (Expressive writing: 29.3%; Control writing: 23.5%). Acceptability measures showed that women who wrote expressively rated the materials/task both more positively and more negatively than those in the control writing group, and qualitative comments revealed that women enjoyed the writing and/or found it helpful even when it was upsetting.

Conclusions

The feasibility of offering expressive writing as a universal self-help intervention to all postpartum women 6 to 12 weeks after birth in the HABiT trial was low, but the expressive writing intervention was acceptable to the majority of women who completed it.

Trial registration

ISRCTN58399513, 10/09/2013.