Many women retain water weight the week before their period due to fluctuating hormones. Fluid retention may reach its peak the first day of your actual period, before subsiding for that cycle.
"With this type of fluid retention, the breasts can get really tender and some women get belly fullness," says Dr. Mack. You might also notice swelling in your face, legs and arms in the days leading up to your period.
Pregnancy can cause you to gain water weight, especially as you get closer to your due date. You may see swelling in your hands, feet, or ankles. Hormones are partly to blame, but your growing baby also puts strain on your blood vessels.
"With pregnancy, you have a big belly so the [pressure causes] the fluid to go out into the tissues, and it has trouble getting back into the vessels," says Jennifer Wu, MD, an ob-gyn at Lenox Hill Hospital in New York City.
If your only symptom is swelling, it's probably normal (although the weight may not all come off the minute you deliver your baby). If you have sudden swelling that hurts, you may have developed a blood clot (especially if the problem is only in one leg) or a spike in blood pressure. Either way, if you have these symptoms, get to a doctor right away.
Hormonal birth control.
Just like there's a connection between pregnancy and menstruation and water retention, hormonal birth control can also sometimes cause water weight.
Both the estrogen and progestin in birth control pills can be culprits, says Dr. Mack. Usually the water weight isn't major and doesn't last long, Dr. Wu adds, but you may want to talk to your ob-gyn about other birth control options.
Cortisol is best known as a "stress hormone," although it's actually much more than that. It's involved in keeping blood sugar levels stable, balancing metabolism, reducing inflammation, and even forming memories.
Water retention as a result of elevated cortisol levels isn't common, but it can happen.