Babies with low birth weights are more likely to die as newborns and have an increased chance of physical or mental development problems.
Low birth weight is also linked to an increased risk of coronary heart disease, type-2 diabetes, and respiratory problems later in life.
Gordon Smith at the University of Cambridge, England, and colleagues report in a Brief Communication that too little of a molecule associated with placenta development (called PAPP-A) produced during the first trimester of pregnancy may underlie low birth weight.
This suggests that low birth weight may already have been determined by the time routine obstetric checks for this eventuality start, at 12 weeks.
Fetal growth may be controlled by a phenomenon called "genetic imprinting". This controls expression of a fetus's genes differently, depending on whether the gene comes from the mother or father.
The father contributes the gene that encodes insulin-like growth factor (IGF).
More IGF means a big placenta that feeds more nutrients to the fetus. The result: a big, healthy baby that increases the chances of a male's genes making it into the next generation.
| Insulin-like growth factor determines placenta size.
It is in the interests of the mother to combat the effect of this gene by expression of her own imprinted genes that control the size of her baby, preventing her from investing all her energy in one birth.
This allows her to reproduce more than once so that as many of her genes as possible survive in subsequent generations.
Now Miguel Constancia and colleagues have directly tested this conflict theory by specifically removing the IGF gene from the placenta of mice.
This allowed contribution of male imprinted genes in the placenta to be analyzed.
They found that the mutants had a smaller placenta and reduced nutrient transport across it, leading to fetuses that are smaller than normal.
As the IGF gene is from the male, they provide the first evidence for the conflict theory in the placenta - the interface between mother and child.
It is far too early to suggest that babies of low birth weight are the unwitting victims of a battle of the sexes.
However, the two studies provide clues to a potential mechanism and evolutionary explanation, argue Benjamin Tycko and Argiris Efstratiadis of Columbia University, New York, USA, in an accompanying News and Views article.