Hi,
There is no problem for you to swim in your 21 weeks of pregnancy. My wife was pregnant at the time of our swimming pool construction. She used to swim till her 30th week of pregnancy. We had asked the gynaecologist about this and he told me that there is no risk for doing this. Swimming is a great exercise and is best for energizing our body. But you should be aware of the water in your pool. This is because swimming pool water contains compounds of chlorine and other minerals. This may cause some allergic problems which is harmful for pregnant women. So it is better to conduct a swimming pool maintenance before you start swimming.
I think lol dont quote me on it
It gets too hot for your baby inside
Hot tubs, Jacuzzis, and hot baths cause your body temp to rise which leads to vasodilation. Vasodilation is when your blood vessels dilate causing a drop on blood pressure which can I'm turn decrease the amount of oxygen getting to baby. Cold water, like in a pool, won't have any effect of your blood pressure and is perfectly safe up until your water breaks.
Its safe. I swim everyday. The only thing that seems to happen is I get tired. So I go swim before bed that way I get a good nights rest.