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.
Gissler
M
M
STAKES (National Research and Development Centre for Welfare and Health), Helsinki, Finland. mika.gissler@helsinki.fi
Hemminki
E
E
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