His is according to research carried out by the University of California, San Francisco. The findings showed that a short interval between pregnancies - six months or less - or a long interval - over 35 months - was linked to low birth weight. There was no such correlation for infants born between six and 35 months after their previous sibling. Low birth weight infants are at increased risk of illness during the first few weeks of life. Women with short interval pregnancies were 71 per cent more likely to have a very low birth weight baby and 43 per cent more likely to have moderately low birth weight baby. Women with long interval pregnancies were 30 per cent more likely to have low or moderately low birth weight babies.