The study included pregnant women delivering at 56 obstetric practices and 15 clinics associated with six hospitals in Connecticut and Massachusetts between September 1996 and January 2000. Exclusion criteria included gestational age greater than 24 weeks at enrollment, mother having insulin-dependent diabetes mellitus, mother speaking neither English or Spanish, and intent to terminate the pregnancy.
A total of 11,267 women were screened and 9,576 (85%) were eligible to participate. All women drinking 150 mg or more of caffeine daily in the prior week (n=718) and a random sample of those drinking no caffeine or less than 150 mg per day (n=2,915), were invited to participate (total n=3,633). Of these women, 2,478 (68.2%) enrolled; 639 declined to participate (17.6%, same percentage by caffeine exposure), 20 (0.6%) were ineligible at time of home interview, 72 (2.0%) miscarried prior to interview, and 424 (11.7%) were lost to follow-up or could not be contacted for interview before the 24th gestational week. Of the 2,478 women enrolled in the study, 187 were excluded from this analysis (70 miscarried, 53 delivered multiple infants, 9 terminated their pregnancies, six experienced stillbirths, five withdrew from the study, and 44 could not be traced) leaving 2,291 women who had a singleton birth.
Based on gestational age at interview, caffeine consumption and random selection, these 2,291 women were placed into one of three subgroups: 1) a telephone group, 2) an intensively monitored group (if <= 13 weeks gestation), or 3) a biochemically monitored group. The telephone group (n=1,882) was interviewed once during pregnancy at a randomly assigned week (20, 28, or 36) of gestation. The intensively monitored (n=164) and biochemically monitored (n=245) groups were interviewed at all three time points; the intensively monitored group also provided urine samples at each time point while the biochemically monitored group provided a urine sample at one randomly selected time point. Of those 2,291 women, 1,672 women provided an umbilical cord blood sample, 268 women gave a urine sample at the home interview and at least one additional urine during pregnancy, 141 women gave a urine sample at 20 weeks gestation, 144 women gave a urine sample at week 28, 134 women gave a urine sample at week 36, and 99 women provided a urine sample at delivery.
We compared women who gave a cord blood sample with those who did not with regard to age, race, education, parity, marital status, and alcohol, smoking and caffeine consumption during the first and third trimesters. The only differences were for education and alcohol consumption. Women with less than a high school education were more likely to give a sample than those who were more educated. Conversely, heavier alcohol consumers (> 1 absolute ounce/day) were less likely to provide a cord blood sample, compared to women who drank less alcohol.
Because of the sampling strategy, more women provided information on reported intake during the first and third trimesters (n=2292 and 2157 respectively). All data from each reporting period were used for each analysis to maximize the information collected.