Among 565 women in the Tanzanian district of Muheza, lead author Patrick Duffy, a malariologist at Seattle Biomedical Research Institute, noticed that a mutation in a foetal gene called sFlt1 seemed to spare the foetus from malaria, yet only during a mother's first pregnancy.
For first-time mothers, all of 75 children with two copies of the mutation were delivered alive.
Yet among first children with another version of sFlt1, common in places without malaria, 10 percent did not carry to term.
These children were also more than twice as likely to be born underweight as children with the protective version of the gene.
Duffy said that this version of sFlt1 seems to cut down on maternal inflammation and protect first-borns.
After one or two more pregnancies, mothers develop antibody resistance to placental malaria, allowing children without the non-protective mutation to survive.
"This is the first resistance gene identified for any infectious disease that functions in [the womb]," New Scientist quoted him, as saying.
The study is published in Proceedings of the National Academy of Sciences.