Abstract: In their twin study of concentrations of fibrinogen and other haemostatic proteins, Marlies de Lange and colleagues (Jan 13, p 101)1de Lange M Snieder H Ariëns RAS Spector TD Grant PJ The genetics of haemostasis.Lancet. 2001; 357: 101-105Summary Full Text Full Text PDF PubMed Scopus (253) Google Scholar show greater concordance in monozygotic than dizygotic twins, which they interpret as showing significant heritability. However, there is growing evidence that environmental factors acting during intrauterine life are also important.2Martyn CN Meade TW Stirling Y Barker DJP Plasma concentrations of fibrinogen and factor VII in adult life and their relation to intra-uterine growth.Br J Haematol. 1995; 89: 142-146Crossref PubMed Scopus (93) Google Scholar Twin studies assume that the intrauterine environment of monozygotic and dizygotic twins is similar. Because the intrauterine environment of monozygotic twins (among whom around 66% share a placenta) differs substantially from that of dizygotic twins, these heritability estimates might not be correct.3Phillips DIW Twin studies in medical research: can they tell us whether diseases are genetically determined?.Lancet. 1993; 341: 1008-1009Abstract PubMed Scopus (199) Google Scholar Some measure of these intrauterine influences can be estimated by comparing concordance rates in monozygotic twins who share a placenta (monochorionic) with those in monozygotic twins who do not (dichorionic). In the absence of fetoplacental influences, correlations should be similar in the two types of twin. In a large prospective study in east Flanders,4Loos R Derom C Vlietinck R Derom R The East Flanders prospective twin survey (Belgium): a population-based register.Twin Res. 1998; 1: 167-175PubMed Google Scholar Belgium, we measured plasma concentrations of fasting fibrinogen in a sample of young adult female twins (mean age 25 years [SD 4·6]). At birth, zygosity was determined by sequential analysis of sex, fetal membranes, blood-group, and DNA fingerprinting, and placentation of the twins by histological examination or dye injection. The overall within-pair correlation coefficient for fibrinogen concentrations was 0·52 in monozygotic twins, compared with 0·31 in dizygotic twins. However, we saw substantial differences in correlation according to chorionicity. The within-pair correlation for monozygotic monochorionic twins was significantly lower than that of monozygotic dichorionic twins before and after adjustment for factors influencing fibrinogen concentrations (body-mass index, smoking, and age [table]).TableDifferences by chorionicity in monozygotic twinsMonochorionicDichorionicNumber of twin pairs6756Age (years)25·8 (4·5)24 (4·8)Body-mass index (kg/m2)22·3 (3·9)22·1 (3·7)Current smoking (%)30·227·2Fibrinogen (mg/dL)239·8 (1·2)235·1 (1·2)Within-pair correlation0·32*p<0·01.0·73*p<0·01.Adjusted correlation0·32*p<0·01.0·74*p<0·01.Values are mean (SD) unless marked otherwise.* p<0·01. Open table in a new tab Values are mean (SD) unless marked otherwise. Fetoplacental influences seem important determinants of plasma fibrinogen concentrations. Our results suggest that heritability estimates in twin studies need to take account of intrauterine influences.