To my understanding only carb intake and resulting insulin causes fat gain. (idmprogram.com)
This is a function of survival, a protection against indiscriminate muscle catabolism if you, for some reason, had to live on a diet of only carbs (as our ancestors may have done at times with vegetables back in the day). (blog.fitplanapp.com)
So let's recap: expectant mothers of multiples have lots more nutrients to get with a moderate amount of extra calories to get them with; we need loads more protein, but we're craving only carbs; the list of foods we can actually eat is shorter than ever due to food aversions and food safety considerations, but our energy to plan and execute appropriate meals is limited. (twiniversity.com)