A woman's diet in early life has more impact on her baby's birth weight than the food she eats as an adult, researchers say.
The surprise finding suggests that you are what your mother ate, and that a woman's diet in her adult life has less influence on her baby's health than previously thought.
Prof Christopher Kuzawa at Northwestern University in Illinois said that women's bodies seemed to "buffer" the supply of nutrients to their unborn babies, meaning that foetuses were partly protected from changes in women's diets.
Kuzawa advised pregnant women to follow a healthy diet, but said they need not worry about every calorie because their health and diet as a toddler could be more important for their baby.
"There is some good news here for expectant mothers. Although there certainly are some harmful things to avoid during pregnancy, and some supplements to take to make sure some important bases are covered, the mother's body seems to do a good job of buffering overall nutritional supply to her growing baby," he said.
"Within the bounds of a healthy balanced diet, the overall quantity of food that a mother eats is unlikely to have large effects on her baby's birth weight," he added.
The findings emerged from a 30-year study that followed more than 3,000 pregnant women in the Philippines whose children have now begun to have babies of their own.
Kuzawa said that while there was good evidence that unborn children benefit from their mothers taking extra folate and that they are harmed by toxins such as lead, mercury, excessive alcohol and bisphenol A, which is used to make some plastics, the picture was less clear on the roles of calories, protein, fat and carbohydrates.
The study suggested that a mother's diet as an adult had no effect on her baby's birth weight. Far more important were the mother's health and nutrition as a baby and toddler, and even the grandmother's diet when she was pregnant with the baby's mother.
The work raises the prospect that a person's health at birth is governed by a long history of health and nutrition going back more than a generation.
"Our findings add to growing evidence that the key to optimising the health of future generations is to promote good nutrition and health of the infants and young children who will be the next generation of mothers," Kuzawa said.
Low birthweight is associated with a raft of health problems, including raised blood pressure and cholesterol levels, and increased insulin resistance, an early sign of diabetes. People who are born light also face a greater risk of heart disease.
While previous work has emphasised that people are shaped by the time spent in their mother's womb, the latest work throws fresh light on what matters for a healthy start to life. "The foetus's experiences during those nine months are akin to 'memories' of the mother's past nutrition and health, rather than cues of what she is eating during pregnancy," said Kuzawa.
To follow up their study, the researchers looked at medical records from 84 mothers who had babies at a birthing clinic in a rural area of the Philippines 15 miles from Manila. In particular, they looked at whether the mothers' leg length was related to their baby's birthweight. Because legs grow quickly, they are a good indicator of a healthy diet in early childhood.
In research that has yet to be published Kuzawa found that mothers with longer legs had heavier babies. The finding supports the idea that a woman's childhood diet has an impact on her baby's birth weight. He described the study at the American Association for the Advancement of Science annual meeting in Chicago.
"An important question raised by this research is where the "memory" of early nutrition resides in our bodies," Kuzawa said. "One promising possibility that we are investigating at Cebu are 'epigenetic' changes. Epigenetic changes involve chemical changes to DNA that influence which genes can be turned on and which are silenced. You can think of them as volume controls for genes."
