General Swimming and Swimming in Pregnancy
For females of all ages, who swim in cold water, I never recommend staying in the water for more than 30 minutes even if your body has adjusted to the water temperature. The cold water is entering your uterus. If the water is warm no more than 45 minutes, please. The same applies to those who are pregnant.
If you are pregnant, that cold water enters your uterus this will end up with you having a cold uterus and the baby cannot survive in a cold environment. You need a warm uterus for the pregnancy to be able to continue. Hence the term bun in the oven. The warm uterus means a happy baby.
For menstruating females, please do not go swimming whilst on period even if you wear a tampon. Your uterus is vulnerable and can lead to issues with your menstruation and later fertility.
If your swimming instructor or medical doctor advises you otherwise, remember that sickness does not happen overnight.