A mother's diet around the time of conception can permanently influence her baby's DNA, research suggests.
Animal experiments show diet in pregnancy can switch genes on or off, but this is the first human evidence.
The research followed women in rural Gambia, where seasonal climate leads to big differences in diet between rainy and dry periods.
It emphasises the need for a well-balanced diet before conception and in pregnancy, says a UK/US team.
Scientists followed 84 pregnant women who conceived at the peak of the rainy season, and about the same number who conceived at the peak of the dry season.
Nutrient levels were measured in blood samples taken from the women; while the DNA of their babies was analysed two to eight months after birth.
Lead scientist Dr Branwen Hennig, from the London School of Hygiene & Tropical Medicine, said it was the first demonstration in humans that a mother's nutrition at the time of conception can change how her child's genes will be interpreted for life.
She told BBC News: "Our results have shown that maternal nutrition pre-conception and in early pregnancy is important and may have implications for health outcomes of the next generation.
"Women should have a well-balanced food diet prior to conception and during pregnancy."
Experiments in mice show diet during pregnancy can have a life-long impact on the genes of offspring.
For instance, the coat colour of a mouse is influenced by its mother's diet.
These are known as "epigenetic effects" (modifications to DNA that turn genes on and off).
One such modification involves attaching chemicals called methyl groups to DNA.
Infants from rainy season conceptions had consistently higher rates of methylation in all six genes studied, the researchers found.
These were linked to various levels of nutrients in the mother's blood.
But it is not yet known what the genes do, and what effect the process might have.
Co-researcher Dr Rob Waterland of Baylor College of Medicine in Houston said the findings, published in Nature Communications, were a proof in principle that a mother's diet can have epigenetic effects.
The research was showing that a mother's nutrition "can leave permanent marks on her child's genome on all the cells of the body", he told BBC News.
Co-author Andrew Prentice, professor of international nutrition at the London School of Hygiene and Tropical Medicine, added: "Our ultimate goal is to define an optimal diet for mothers-to-be that would prevent defects in the methylation process."