I was losing weight before I got pregnant (earlier than I'd hoped, which was great because we had been TTC for ages, but meant I was only about halfway to my weight target - which was disappointing as I hoped to have a lower BMI when I got pregnant).
I continued on my mostly healthy eating (nothing drastic) when I got pregnant. I also suffered from morning all day sickness for the first four months. So I lost quite a bit of weight off of my booking in weight. I've put a little back on since my sickness eased off and my appetite for unhealthy food increased but I'm still below my booking in weight at 24 weeks.
At my recent midwife appointment, she said that she wasn't worried that I had lost some weight due to morning sickness, but that she wouldn't like to see me keep losing weight. She said it was fine to try and maintain my current weight.
You need to think seriously about why you want to lose weight during pregnancy. As opposed to say, following healthy eating and gentle exercise advice which is recommended for everyone.
I understand that if you are overweight then you might feel the need to be lighter to be 'healthier' but the standard advice on healthy eating and exercise will make you healthier anyway (if you had previously been unhealthy). You don't need to do anything drastic to be healthy, and there is a danger that a crash diet will make you more unhealthy.
There has been a study that looked into malnutrition in pregnancy with obesity in children. Unfortunately it isn't clear what they meant by malnutrition, whether the mums weren't eating the right food, or whether they weren't eating enough. Anyway, the theory is that the baby's developing body is given the information that food is hard to come by and so switches on genes to make it take more calories from food, and therefore more likely to become overweight later.
In case I have misinterpreted this research, feel free to look it up yourself Can your pregnancy diet make your child fat
I couldn't work out what they suggest, given their research, what mothers should do. It seems to suggest that mother's intake of carbohydrates in pregnancy shouldn't be too low. So definately no Atkins! I think you need to go with a balanced diet with all food groups. I don't want to scare you about not eating enough, but it would be a shame to push yourself to lose weight in pregnancy because you've experienced weight issues, only to make your growing baby into a calorie munching machine who may struggle with food too.
Do speak to your midwife about how much you should eat and whether losing weight is desirable, particuarly if you are concerned about being overweight and complications of a raised BMI in pregnancy. That said, you might find that you lose a little weight on just eating healthily and gentle exercise. Women are different, some put on lots of weight, others just a little, some stay the same weight and others may lose some, and they can all have healthy pregnancies.
I do sympathise with you wanting to get your body in shape for pregnancy, but you really have to try and do it in a gentle way and not punish your body. Best wishes xx