The study cohort included healthy women between 18
and 40 years of age with singleton pregnancies who received
prenatal care between May and August 2003. Those with sickle cell
disease, major congenital or current illnesses were excluded.
Among the women eligible for the study a subset was selected by a
computer-generated random list of numbers. Recruitment, enrollment,
and data and specimen collection occurred during a single
outpatient visit at one of two distinct times during pregnancy: the
first prenatal visit (4–24 weeks of pregnancy) and the last month of
pregnancy (36–41 weeks).