Almost one in four Australian women are obese when they become pregnant and alarming new evidence shows their babies' health is at risk as a result, The Courier-Mail reported Sunday.

Obese women are more likely to have miscarriages and malformed babies and 50 percent of them will have to give birth by caesarean section – often weeks before the child is due.

One in four of the babies born to obese women weigh over 8.8 pounds and their weight combined with the mother's obesity could cause problems during the birth, Sydney gynaecologist Dr Andrew Zuschman said.

Zuschman studied the pregnancy outcomes of obese women between June 2008 and July 2009.

He said the large babies born to obese women often had their shoulders stuck in the birth canal and this could lower the baby's oxygen levels and leave them with injuries or nerve problems.

"You can prevent all of these pregnancy complications by a woman optimizing her weight before she falls pregnant."

He said obese pregnant women also faced serious health implications.

They are twice as likely to develop gestational diabetes, and 2.6 times more likely to develop severe hypertensive disease in pregnancy compared to normal weight women.

It is these two risk factors combined with birth complications caused by their weight that lead to 50 percent of obese pregnant women having a caesarean.

Babies born to obese pregnant women with diabetes have a higher than normal rate of unexplained still births after 38 weeks of the pregnancy.

For this reason, many women in this category were advised to have a caesarean at 38 weeks.

"All of these things lead to more babies that need to go to the special care nursery to have oxygen and blood sugar monitoring, it all develops a whole cascade," he said.

Zuschman said women in this group were advised to keep their weight gain to between 13.2 and 17.6 pounds during pregnancy compared with the normal range of 26.4 to 30.8 pounds.

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