The institute issued advice last week on weight management before, during and after pregnancy.
Women who are obese when pregnant are more likely to develop impaired glucose tolerance, gestational diabetes, pre-eclampsia and thromboembolism, it says. They are also more likely to have a miscarriage or die during pregnancy.
Lucilla Poston, professor of maternal and fetal health at King's College London, helped develop the guidance. She said GPs and women need to be as aware of the risks associated with obesity in pregnancy as they are of the dangers of smoking while pregnant.
Speaking to GP at the launch of the guidance, she said: 'Nobody minds talking about smoking now and we have got to have the same attitude about obesity.
'People all know about smoking in pregnancy and having small babies and you ask them about having a BMI over 25 and they say they do not know about the problems.'
NICE is advising women not to diet during pregnancy, but to try to achieve a healthy weight before becoming pregnant.
But NICE decided not to publish advice on how much weight women should gain during pregnancy. It said there was as yet insufficient evidence.
RCGP chairman Professor Steve Field said the media often presented conflicting information about healthy weight during and after pregnancy. 'I welcome any guidance that can offer prospective and new mothers comprehensive direction and clarity,' he said.