Altered blood pressure course during normal pregnancy and increased preeclampsia at high altitude (3100 meters) in Colorado

OBJECTIVE: Our purpose was to determine the case incidences of preeclampsia at low and high altitudes and whether maternal blood pressure course durin g pregnancy differs between low and high altitudes. STUDY DESIGN: This was a retrospective cohort study of pregnancies in socio demographically matched communities at low and high altitudes in Colorado; each community had a small hospital served by family practitioners and was located >100 miles from major urban areas. Included were consecutive single ton pregnancies of women without chronic disease that resulted in live-born infants at >28 weeks' gestation during an 18-month period (n = 116 at 1260 m, n = 93 at 3100 m). Clinic and hospital medical records were searched an d data pertaining to hypertensive complications of pregnancy and serial blo od pressure measurements were abstracted. RESULTS: Despite similar maternal risk factors, the case incidences of pree clampsia were 16% at 3100 m and 3% at 1260 m. As in sea-level pregnancies, mean blood pressure fell until week 20 in normotensive pregnancy at 1260 m. Mean pressure rose linearly, however, in normotensive women at 3100 m and in women with preeclampsia at both 1260 m and 3100 m. High altitude acted i ndependently of known risk factors and yielded an odds ratio for preeclamps ia of 3.6 (95% confidence interval 1.1-11.9). Birth weight was 285 g lower at 3100 m despite similar gestational ages. CONCLUSION: The normal pregnancy-associated fall in blood pressure was abse nt at 3100 m, even in women who remained normotensive. The incidence of pre eclampsia was increased at high altitude. Residence at high altitude interf eres with the normal vascular adjustments to pregnancy, increasing the inci dence of preeclampsia, and is perhaps analogous to other conditions that de crease uteroplacental oxygen delivery.