The danger of overmatching in studies of the perinatal mortality and birthweight of infants born after assisted conception.
OBJECTIVE
To examine the impact of gestation length and plurality on short-term outcome of in vitro fertilization (IVF)-children.


METHODS
A register study using the Finnish Medical Birth Register for 1991-1993 (N = 194 383 newborns, of which 1335 were IVF-newborns).


RESULTS
For IVF-newborns, a high proportion of multiple births (27%, odds ratio (OR) 19.67 compared with non-IVF births), low birth weight infants (30%, OR 7.94), and perinatal deaths (2.9%, OR 4.17) was found. The mothers' background did not explain the increased risks. After adjusting for gestation length and/or plurality, the odds ratios decreased significantly.


CONCLUSIONS
Because of the high risk of plurality and premature births, matching the control group by gestation length and/or the number of multiples births may yield misleading results on the total health impact of IVF, and therefore it should be avoided.
STAKES (National Research and Development Centre for Welfare and Health), Helsinki, Finland. mika.gissler@helsinki.fi
M
M
Gissler
E
E
Hemminki
D001724:Birth Weight; D005260:Female; D005307:Fertilization in Vitro / Q000009:adverse effects*; D005387:Finland; D005865:Gestational Age; D006801:Humans; D007226:Infant Mortality; D007231:Infant, Newborn; D009035:Mothers; D016017:Odds Ratio; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000208:ethnology / Q000706:statistics & numerical data*; D011446:Prospective Studies; D013673:Technology Assessment, Biomedical