A decline in progesterone around age 38 in women can lead to unexplained weight gain.

“Now when a woman hits about 38 and it's a very exact age because this is when i see it happening the most at 38 something starts to happen and it's usually the progesterone starts to drop and this is when women will just suddenly and and men too i'm sure but women specifically you just suddenly you know you put on a couple pounds when you stayed the same weight for years and suddenly it's like oh i've gained a couple pounds so you start working out harder eating better watching what you eat but the weight doesn't go away”