Seven pounds may not seem like much, but new research suggests they're critical to a healthy second pregnancy.

A study led by researchers at Harvard and in Sweden found that women who gain seven pounds or more between pregnancies increase their risk of health complications.

Doctors have long advised obese women who plan to become pregnant to lose weight.

But this is the first study to suggest that women who remain at a healthy weight can put themselves at risk of diabetes and high blood pressure.

The researchers studied 150-thousand Swedish women.

Their study is published in the British medical journal, The Lancet.