I have come across too many women who use pregnancy as an excuse to enjoy a "fat period" in their lives. This is the worst time to have this mindset. Fitness and a healthy diet should be the forefront of every pregnancy. Women who approach pregnancy with the intent of letting themselves go, end up getting fat and staying fat.
When a woman gets pregnant she thinks she can binge on all sorts of foods that were previously off-limits? Why would she do this?
Exhibit A
I've also seen the mom who is fit and pregnant - a rarity - but I've seen it, so I know it's possible. Ladies, stop using pregnancy as an excuse to get fat. It doesn't help you, your children, and especially your husband.
Exhibit B
Can genes be turned on and off in cells?
https://ghr.nlm.nih.gov/primer/howgeneswork/geneonoff
By no means would I call myself an expert...