Table 1.

Twin studies of gestational age and preterm birth

Heritability estimate	Authors	Study type	Variable measured
36% (maternal)	Clausson et al., 2000	Twin study investigating concordance and correlation between twin mothers of differing zygosity (potential confounding from fetal genome)	Gestational age (no distinction between spontaneous and medically indicated onset of labor)
14% (maternal) 11% (fetal)	Lunde et al., 2007	Parent–offspring study using path analysis and maximum likelihood principles and Pearson's correlation coefficients for different familial relationships	Gestational age (births by Caesarean section were excluded as these births are more likely to be medically indicated)
34% (maternal) Negligible (paternal)	Kistka et al., 2008	Extended twin design investigating the correlations between monozygotic twins and first‐degree relatives reporting on the gestational age of their first‐born child (potential confounding from fetal genome)	Gestational age (no distinction between spontaneous or medically indicated onset of labor)
44% (maternal) 33% (fetal)	Plunkett et al., 2009	Family study with mother–infant pairs using segregation analysis and estimating the parameters using maximum likelihood method (potential confounding between maternal and fetal estimates)	Gestational age (spontaneous onset of labor)
27% (maternal)	Treloar et al., 2000	Twin study (potential confounding from fetal genome)	Any preterm birth (no distinction between spontaneous or medically indicated onset of labor)
20.6% (maternal) 13.1% (fetal)	York et al., 2013	Extended twin‐sibling design investigating the offspring of twins, full siblings, and half‐siblings using structural equation modeling. (excluded multifetal pregnancies)	Gestational age (exclusions for some medical reasons including preeclampsia and placental abruption)
25% (maternal) (29% for spontaneous; 13% for indicated) 5% (fetal) (negligible for spontaneous; 14% for indicated)	Svensson et al., 2009	Family study investigating children of siblings using an alternating logistic regression model	Preterm birth (live birth at <37 weeks gestation) (data were analyzed as a combined group including spontaneous and medically indicated onset of labor and also as two separate groups)
14.21% (maternal) 6.77% (paternal) 13.33% (narrow‐sense) 24.45% (broad‐sense)	Wu et al., 2015	Family study assessing the correlation of gestational age between offspring and full/half siblings and a regression analysis in parent offspring pairs; narrow‐sense heritability was calculated as 4 times the paternal half‐sib correlation and broad‐sense heritability was calculated as the narrow sense heritability plus the full sibling correlation (representing the dominance variance) (not able to control for environmental risk factors)	Gestational age (excluded births that were unlikely to have a spontaneous onset of labor)