Eating for two during pregnancy may lead to a weight problem because of changes to the digestive system, new research suggests.
Pregnancy causes hormone-driven growth of the intestines that allows more energy to be extracted from the same amount of food, a study shows.
The discovery, made in fruit flies, could explain why some women struggle to lose weight after giving birth, say scientists.
They point out that the same metabolic responses seen in fruit flies should also occur in humans.
A "juvenile hormone" in the flies was found to trigger growth of the intestines soon after mating and also stimulated fat storage.
The hormone acted in a similar way to human thyroid hormones, which regulate the body's energy demands, the researchers pointed out.
Lead scientist Dr Irene Miguel-Aliaga, from the Medical Research Council (MRC) Clinical Sciences Centre in London, said: "Previous studies have shown that eating for two during early pregnancy is unnecessary.
"Our research suggests that this is because the digestive system is already anticipating the demands that the growing baby will place upon our body."
Co-author Dr Jake Jacobson, also from the MRC Clinical Sciences Centre, added: "Many of the fly genes that we studied exist in humans. Flies also utilise and store fat like we do, and their metabolism is controlled by similar hormones.
"Some women find it difficult to lose weight after pregnancy, and we may now have found a biological reason for this."
The metabolic changes appeared to have a role in determining fertility, said the scientists writing in the journal eLife. Female flies produced fewer eggs when the juvenile hormone was prevented from enlarging their intestines.
Human mothers are thought to experience similar intestinal growth after becoming pregnant.
If her hormone levels fail to normalise after birth, a mother's enlarged intestine might continue to absorb more calories, said the researchers.
Dr Joe McNamara, head of population and systems medicine at the MRC, said: "This research points to a new scientific explanation why eating for two during pregnancy is not necessary, and may even be harmful, as a growing body of 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."
Dr Alison Tedstone, chief nutritionist at Public Health England, said: "It is even more important to eat a healthy, balanced diet during pregnancy to ensure your baby gets the range of nutrients they need to grow and develop.
"You don't need to eat for two, even if you are expecting twins or triplets. Evidence shows that expectant mothers may need to eat more in the last few months of the pregnancy, around 200 extra calories a day."
A bagel, a teacake or a couple of slices of toast amount to around 200 calories.