Pregnant muslim women who fast during Ramadan are likely to have smaller babies who will be more prone to learning disabilities in adulthood, according to new research.
Scientists in the United States also found that the women were 10 per cent less likely to give birth to a boy if they had fasted during Ramadan.
The trend was clearest if the fasting was done early in the women's pregnancy, and during the summer months, when long hours of daylight called for them to go longer without food.
Ramadan is the ninth month of the Islamic calendar and a time when Muslims across the world fast from dawn until sunset. Three in four Muslim pregnancies overlap with Ramadan and surveys indicate that the majority of pregnant Muslims observe the fast. This year, it falls between 11 August and 9 September.
Although pregnant women may request an exemption from fasting, they are expected to "make up" the fasting days missed during pregnancy after their baby is born.
Previous research has suggested that this requirement may discourage pregnant women from seeking the exemption, since they do not want to be the only member of their household fasting. Some Muslims also interpret Islamic law as requiring pregnant women in good health to fast.
Since fasting during Ramadan is one of the five pillars of Islam and is a central part of Muslim culture, many women may fear a loss of connection with their communities or would feel guilty if they did not observe Ramadan.
The study, which used census data from the US, Iraq and Uganda, also discovered long-term effects on the adult's health and his or her future economic success.
Douglas Almond, of Columbia University, and Bhashkar Mazumder, of the Federal Research Bank of Chicago, the authors of the research, concluded: "We generally find the largest effects on adults when Ramadan falls early in pregnancy.
"Rates of adult disability are roughly 20 per cent higher, with specific mental disabilities showing substantially larger effects. Importantly, we detect no corresponding outcome differences when the same design is applied to non-Muslims."
Sheikh Ibrahim Mogra, an imam from Leicester, said sharia (Islamic law) would never expect a woman to fast if it had an adverse effect on either the mother or child.
"Sharia would not want the mother to unnecessarily burden herself," he said. "There's no point being needlessly brave. When Ramadan falls during the winter months, more women will fast. But when it falls during the summer, when you might have to fast for 16 or 17 hours, it is understandable that fewer mothers will be fasting."
Nusrat Hussain, a mother of four from Ilford, fasted during her second pregnancy, which fell during Ramadan.
"It was something I wanted to do. But I started to feel dizzy and my husband and I decided I should stop. There's no Islamic compulsion to fast if you're pregnant but I think people do sometimes feel pressured within some cultures," she said.