Study participants were pregnant women attending prenatal care at clinics affiliated with Swedish Medical Center in Seattle, Washington and enrolled in the Migraine and Pregnancy Study, a pregnancy cohort study designed to investigate the relationship between migraine, headache symptoms before and during pregnancy, and the risk of preeclampsia . The study population for this report is from the first 500 participants who were enrolled (consecutively) and were interviewed during the period of April 2009 and December 2010. Women were ineligible if they initiated prenatal care after 20 weeks gestation, were younger than 18 years of age, did not speak and read English, did not plan to carry the pregnancy to term, or did not plan to deliver at Swedish Medical Center. Participants completed a questionnaire administered by trained interviewers (supervised by neurologist and maternal fetal medicine clinicians) at enrollment. Participants were asked to provide information pertaining to their medical history, pre-pregnancy weight, general health, pregnancy-related symptoms, socio-demographic, and lifestyle characteristics. The interview included a structured migraine assessment questionnaire (adapted from the deCODE Genetics migraine questionnaire (DMQ3)  (Additional file 1) and an assessment of disability associated with headaches experienced before and during pregnancy by Migraine Disability Assessment (MIDAS) Questionnaire . In previous validation study, using a physician-conducted interview as an empirical index of validity, the deCODE Migraine Questionnaire (DMQ3) diagnosed migraine with a sensitivity of 99%, a specificity of 86% and a kappa statistic of 0.89 . The detailed migraine-specific questionnaire contained questions addressing age at migraine onset, physician diagnosis of migraine, family history of migraine, details about migraine attacks and medication used.
Headache classification was determined using the ICHD-II criteria established by the International Headache Society (IHS) . “Definitive Migraine” (IHS category 1.1 or 1.2) was defined by at least five lifetime headache attacks (criterion A) lasting 4–72 hours (criterion B), with at least two of the qualifying pain characteristics [unilateral location (criterion C1), pulsating quality (criterion C2), moderate or severe pain intensity (criterion C3), aggravation by routine physical exertion (criterion C4)]; at least one of the associated symptoms [nausea and/or vomiting (criterion D1), photo/phonophobia (criterion D2)]; and not readily attributable to another central nervous system disorder or head trauma (according to subject self-report) (criterion E). “Probable Migraine” (IHS category 1.6) was designated if all but one of the definitive migraine criteria were fulfilled, excluding headaches attributable to another disorder. Finally, any migraine was defined as the group with either definitive migraine or probable migraine combined.
The procedures used in the study were in agreement with the protocol approved by the Institutional Review Board of Swedish Medical Center (Swedish IRB # 008567). All participants provided written informed consent.
Frequency distributions of sociodemographic, reproductive, medical and behavioral factors among groups defined by ICHD-II (any migraine, no migraine) and the cohort were compared using means (± standard deviation (SD)) for continuous variables and counts and percentages for categorical variables. Bivariate differences in characteristics associated with definitive and probable migraine headaches were determined using Chi-square test (or Fisher’s exact test) for categorical variables and Student’s t-test for continuous variables.
The self-reported physician migraine diagnosis was compared with the ICHD-II based diagnosis. The sensitivity and specificity as well as positive predictive value and negative predictive value of the self-reported diagnosis were assessed. Concordance also was determined by estimating the value of Cohen’s kappa coefficient . All analyses were performed using Stata 9.0 (Stata, College Station, TX) statistical analysis software. All reported p-values are two-tailed. The 95% confidence interval (CI) for the prevalence estimate of migraine was determined using previously described methods .