Birth Weight

Birth weight is one of the most-studied early-life measurements in medicine, because low birth weight has long been observationally linked to higher adult risk of type 2 diabetes, high blood pressure and heart disease. What genetics adds is a way to ask whether that link is causal, and if so, where it actually comes from — the mother's body, the child's own genes, or both.

The genetic correlation with adult disease is real, not just observational

Horikoshi et al. 2016 combined data from 153,781 people of multiple ancestries and found 60 loci associated with birth weight, together explaining about 15% of its variation. Using genetics alone — not comparing outcomes in the same people, but comparing which genes influence birth weight against which genes influence adult disease — they found strong inverse genetic correlations with adult systolic blood pressure, type 2 diabetes and coronary artery disease. In plain terms: many of the same genetic variants that push birth weight down also push adult cardiometabolic risk up. This page holds 10 of that study's loci.

Whose genes actually cause it — the mother's or the child's

A lower birth weight being linked to higher adult blood pressure could mean two very different things: an adverse environment in the womb permanently changing how a body develops (the traditional "fetal programming" explanation), or the same genetic variants simply being inherited by the child and acting on their own blood pressure later, independent of the womb. Warrington et al. 2019 was built specifically to tell these apart, combining a person's own birth weight (n = 321,223) with birth weight as reported by mothers about their children (n = 230,069 mothers) and separating direct fetal genetic effects from indirect maternal ones with structural equation modelling and Mendelian randomization. It found 190 independent signals, 129 of them new. This page holds 17 of that study's loci — 10 for a person's own birth weight, 7 for offspring birth weight as reported by mothers.

Its central finding is specific and worth stating precisely: a mother's own blood-pressure-raising genetic variants do reduce her baby's birth weight — that part is a real maternal, indirect effect. But it is only the copies of those same variants that the child itself inherits that go on to raise that child's own blood pressure as an adult. Used as a natural experiment, a mother's birth-weight-lowering genotype showed no evidence of causally raising her child's blood pressure through the womb environment on its own. The paper's own conclusion: the birth weight-blood pressure link runs through shared genetics, not through intrauterine programming.

An imprinted gene among these variants

rs234864, one of this page's variants, sits at KCNQ1 — one of the most well-established imprinted regions in the human genome, where only the copy of the gene inherited from one particular parent is normally active. The same 2016 study found that birth-weight associations generally are enriched in known imprinted regions, which is exactly the kind of biology a parent-of-origin-sensitive trait like birth weight would be expected to involve.