I have been researching swimming whilst pregnant and came across this article which is a good short read: http://www.sportcentric.com/vsite/vfile/page/fileurl/0,,4716-1-1-84574-0-file,00.pdf
The main thrust of the advise seems to be that swimming is good, particularly to counterbalance any postural changes that may occur by strengthening your chest and back muscles. This is important because the body’s centre of gravity changes dramatically as you gain weight and rather than let your muscles just adjust in an unbalanced posture you need to keep them healthy and strong by exercising.
For someone carrying a baby the main benefit of swimming has to be getting into the water and feeling as though you are weightless. The buoyancy force from the water pushing you up makes it appear that you weigh only about 10% of your actual weight. There are other benefits too. You’ll be getting an aerobic workout which is gentle on your joints yet strengthening for your muscles. If you’re suffering from sciatica then swimming can help to stretch the back and glut muscles – important in relieving the pressure on the nerve which is being compressed by tight muscles. For those who have water retention and swelling then swimming can help because the pressure of the water can push fluid back into your veins and back into the circulatory system where it needs to be.
A couple of notes of caution for swimming though: just as for sleeping/laying down, exercising on your back may not be a good idea because the weight of the baby can press on major blood vessels (the aorta and vena cava). If you have any concerns you must always speak to your doctor or health care practitioner. Also, if you have Symphysis Pubis Dysfunction you should avoid breaststroke as you need to avoid over-stretching your pelvis and related joints.
To summarise then, swimming is good! It’s non-weight bearing, it can give you an aerobic workout, help relieve sciatica and reduce swelling, lessens pressure on your joints and will help you to relax. Check out your local pool, many will offer aqua classes specifically for pregnant women.