If you can do exercise, that is certainly going to help. Otherwise you really need to just look at the types of food you are consuming. Make sure you are mostly eating healthy fresh produce, are eating across all the food groups, and are stopping when you feel satisfied. It is very hard (especially when everyone seems to try to encourage you to do the oppersite whilst pregnant) but it can be done.
Also when you do fall off the wagon (and you will at times), the importan thing is not to beat yourself up about it, but just get straight back on track the next meal.
I am a big believer in writing down everything you have consumed (food and drink). It keeps you honest with yourself, and you may just be surprised at how much you are actually eating or how much of what you are eating is not very healthy.
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