Experts tested fruit flies and found pregnancy triggers hormone-driven growth of the intestines that allows the body to extract more energy from exactly the same amount of food.
This change happens straight after mating and the same “juvenile hormone” also stimulates fat storage, the Medical Research Council in London found.
Dr Joe McNamara, of the MRC, said: “This research points to a scientific explanation why eating for two during
pregnancy is not necessary, and may even be harmful. Growing evidence indicates that a mother’s diet can impact a child’s propensity to be obese in later life.
“The important next step will be to reproduce these findings in humans.”
The study, in journal eLife, may also help explain why some women struggle to lose weight after giving birth. Experts found if hormone levels fail to normalise after birth, a mother’s enlarged intestine might continue to absorb more calories.
The researchers believe the same metabolic responses seen in the fruit flies should also happen in humans.
Dr David Richmond, president of the Royal College of Obstetricians and Gynaecologists, added: “It is only in the last 12 weeks that women need an extra 200 calories a day.”