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The results of several published studies have suggested that mothers carrying certain polymorphisms in the genes encoding coagulation Factor V, prothrombin or the enzyme methylenetetrahydrofolate reductase (MTHFR) are at  increased risk of having babies that are small for their gestational age, as a result of intra-uterine growth restriction. A Canadian group has now published results that call this conclusion into question [Infante-Rivard, C. et al (2002) N Engl J Med 347, 19-25 (Abstract)]. Comparing 493 babies whose birth weight was below the 10 percentile with 472 matched controls, they found that mothers carrying the Factor V Leiden polymorphism or the prothrombin variant G20210A were not more likely to have low-weight babies; nor were these polymorphisms in the babies themselves associated with low birth weight. The results were more complex for MTHFR polymorphisms: a maternal genotype homozygous for the MRHFR variant A1298C, and a newborn genotype homozygous for the C677T variant, were both associated with a reduced risk of intra-uterine growth restriction. However, the numbers of subjects with these genotypes were small and the associations should not be regarded as firm unless confirmed by further studies. Infante-Rivard et al also carried out a family-based study, in which trios of mother, father and infant were analysed. This study also failed to reveal any association between thrombophilia polymorphisms and low birth weight.

Comment: The main difference between this study and those published previously is its size: previous studies have in general included fewer than 100 cases and controls and so are likely to be less reliable. It is possible that population differences may explain the discrepancy in findings but, until further evidence is available, any association between thrombophilia polymorphisms and intra-uterine growth restriction remains unproven.