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erconditions, particularly pre-eclampsia, lowbirthweight duetohighaltitude doesnotap-
pear to be associated with increased morbidity and mortality for full-term infants.
Various investigations suggest that the mechanism of intrauterine growth retardation at
high altitude is in large part lower uteroplacental blood flow and “relative ischemia” of
the placenta. Three factors seem to explain this decreased placental blood flow: a smaller
maternal blood volume increase, less uterine artery dilatation, and lack of appropriate re-
distribution of blood flow to the uteroplacental circulation. How hypoxia could produce
these changes in unknown but impaired placentation (the process by which the placenta
“invades” the uterus to establish its blood supply) has been suggested. Recent studies
have indicated that hypoxia causes shallower placentation and higher vascular resistance
like that seen in pre-eclampsia. Although this finding needs to be confirmed in normal
highaltitude placentas, this work does question whether during placentation (the first nine
to twelve weeks of pregnancy) hypoxic exposure could be detrimental. A corollary ques-
tion is whether altitude exposure is advisable for lowland women with any suggestion of
impaired placentation or fetal compromise, such as hypertension or pre-eclampsia, at any
time during pregnancy.
In high-altitude residents, intrauterine growth retardation is not linear throughout preg-
nancy. Only after thirty-two weeks of gestation does fetal growth at altitude become ap-
preciably slower than at sea level. Does this observation mean that the otherwise healthy
pregnantlowlanderneednotworryaboutimpairedfetalgrowthataltitudeuntilafterthirty-
two weeks gestation? Or is the stage set for intrauterine growth retardation earlier in the
pregnancy, such as during placentation, so that altitude exposure afterward has no effect
on growth? Is the intrauterine growth retardation of altitude residents even relevant to low-
landers? Without answers to these questions, admonishing all women to avoid altitude ex-
posure throughout pregnancy, as some have done because of concerns about intrauterine
growth retardation, seems premature.
At the present time, no evidence indicates that altitude exposure, at least to moderate
altitudes, increases risk to the healthy pregnant lowlander or her fetus.
Acute Ascent for Pregnant Lowlanders
Clinical and physiologic investigations of pregnant lowlanders ascending to altitude are
conspicuously missing, particularly to altitudes over 8250 feet (2500 m). Thousands of
pregnantwomentraveltothemoderatealtitudeofskiresortsforrecreation.Theabsenceof
any reported adverse effects is reassuring, but the safety of altitude exposure during preg-
nancyhasnotbeensystematicallyevaluated.Isaddingthestressofexercise(skiing)tothat
of hypoxia a cause for concern?
Thefewstudiesavailableareencouraging.Astudyofsubmaximalandmaximalaerobic
work at sea level and 6000 feet (1830 m) after two to four days of acclimatization found
that fetal heart rate responses were not changed from sea level. A study of pregnant in-
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