This BBC News article explains that "if a woman is obese during pregnancy, she has an increased risk of developing serious complications like pre-eclampsia, gestational diabetes, miscarriage and stillbirth. She is also more likely to have a Caesarean section. NICE says women with a body mass index of more than 30 should be encouraged to lose weight before they become pregnant. During pregnancy, losing weight can be harmful to the unborn child, so women are advised to eat healthily and to do gentle exercise."
Unfortunately, in the audio clip from this morning's Radio 4 Today programme, Professor Kelly gives two unhelpful pieces of advice. I'm with him when he says to avoid sugary drinks and drink plenty of water, but when he says that 'healthy eating' for everyone should be based on starchy food and avoiding fatty food I disagree. I believe a healthy diet is logically one that approximates to a Stone Age ('Paleo') diet, and is low in carbohydrates. But the most important advice of all for pregnant women is to avoid salt and salty food. This is the safe way to avoid fluid retention and excess weight gain. And it does not entail going hungry or counting calories.
During pregnancy, because of the hormone changes it brings, a woman is vulnerable to salt/sodium. If during this vulnerable time she eats salty food, her blood volume will rise more than it would if she were not eating salty food. This is because high oestrogen in the blood stream causes the muscles in the walls of the blood vessels to relax and so the blood vessels allow in more sodium and the water it attracts to itself. The veins become swollen.
When the baby is born the hormone levels change again and the oestrogen level goes down. The muscles in the walls of the blood vessels tighten up again and some of the excess salt and water in the blood is excreted. But if the veins were badly overstretched and weakened during the pregnancy, the woman will not return to her former weight. With each succeeding pregnancy, the problem will worsen; the veins will become weaker and more swollen; the blood volume will rise further; she will become more overweight.
See Pregnancy Advice about Salt.
How to lose weight.