What we have learned about the design of randomized trials in pregnancy.

For nearly 30 years the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Maternal-Fetal Medicine Units (MFMU) Network has been conducting randomized trials in pregnant women, many of which have changed clinical practice. Since 1986, the MFMU Network has conducted 29 randomized trials, of which the 17 trials started or completed since 2003 are described here. Study design choices are described including decisions regarding the fundamental questions to be answered and the rationale behind choices of primary and secondary outcomes. Some of the potential pitfalls, particularly relating to bias, that can affect the interpretation of trial results are described along with the mechanisms that the Network has used to avoid or minimize them.