A new study claims that children born to mothers who experience relationship problems during their pregnancies have lower than average intelligence and are more prone to anxiety, scientists said.

The study done at Imperial College London found that women who reported high stress levels in the months before birth had babies who scored lower in mental development tests than mothers with calmer pregnancies, according to the Telegraph.

Fights and violence with a partner were particuarly harmful, according to researchers.

Cortisol, a stress hormone, may be the cause as it crosses the placenta to the unborn child.

Professor Vivette Glover, a fetal development expert at the college, said there was no way to know from her research whether or not the delayed mental development detected at 18 months old would continue later into the child's life, according to the Telegraph.