The association of season and temperature with adverse pregnancy outcome in two German states, a time-series analysis.
BACKGROUND
A seasonality of low birth weight (LBW) and preterm birth (PTB) has been described for most regions and there is evidence that this pattern is caused by ambient outdoor temperature. However, the association as such, the direction of effect and the critical time of exposure remain controversial.


METHODS
Logistic, time-series regression was performed on nearly 300,000 births from two German states to study the association between season and daily mean temperature and changes in daily proportions of term LBW (tLBW) or PTB. Analyses were adjusted for time-varying factors. Temperature exposures were examined during different periods of pregnancy.


RESULTS
Weak evidence for an association between season of conception, season of birth or ambient outdoor temperature and tLBW or PTB was found. Results of analyses of temperature were not consistent between the two states. Different sources of bias which would have artificially led to stronger findings were detected and are described.


CONCLUSIONS
No clear evidence for an association between season of conception, season of birth or temperature and tLBW or PTB was found. In the study of pregnancy outcome different sources of bias can be identified which can potentially explain heterogeneous findings of the past.
Department of Social and Environmental Health, London School of Hygiene and Tropical Medicine, London, United Kingdom. jennyfer.wolf@googlemail.com
Jennyfer
J
Wolf
Ben
B
Armstrong
D005260:Female; D005858:Germany / Q000453:epidemiology; D006801:Humans; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D008297:Male; D011247:Pregnancy; D011256:Pregnancy Outcome / Q000453:epidemiology*; D047928:Premature Birth; D012621:Seasons; D013696:Temperature