The Amateur Swimming Association, the governing body of the sport in the U.K., states that "Swimming is a safe activity for pregnant women," but it also provides a list of guidelines and recommendations. All pregnancies are different, so the only way to check whether it is safe or appropriate for you to swim in the lake during your pregnancy is to ask the advice of your doctor or a medical professional.
Go For It
Your pre-pregnancy health and your health at each stage of your pregnancy will have a bearing on the types of exercise that are best suited to you. If you already are a regular swimmer, you can normally continue to exercise as before. This means that if you enjoyed swimming in the lake before you became pregnant, there is no reason you should not continue to enjoy it throughout your pregnancy, provided that no problems are discovered in your regular ante-natal checks. However, the ASA guidelines state that swimming during pregnancy is not recommended for women with a history of three or more miscarriages, ruptured membranes, early labor, a weak cervix, multiple pregnancies or significant heart or lung disease.
One of the main concerns when lake swimming is the temperature of the water. USA Swimming says the ideal water temperature for pregnant women is between 78 and 84 degrees F, and it cautions that exercising in warmer waters can be detrimental. Your pulse rate is normally higher during pregnancy, so care must be taken to monitor your heart rate and avoid exercising at a pace that leads to your body overheating. Swimming in colder water is a matter of personal preference, and if the lake temperature is something you have become accustomed to as a regular swimmer, it should not be a cause for concern during pregnancy. Other concerns to be aware of include raised blood pressure, swelling in your ankles, lower abdominal pain and uterine contractions or vaginal bleeding. If you experience any abnormal symptoms after swimming or feel light-headed, dizzy, or breathless, or notice an irregular heartbeat during your swim, stop immediately and seek the advice of a medical professional.
Is It True?
There are a number of common myths and misconceptions surrounding swimming during pregnancy, one of which is that lake swimming should be avoided due to the increased risk of infection from dirty water. The Outdoor Swimming Society in the U.K. states that this is a minimal risk that can be moderated with common sense. Water cleanliness data can be obtained from local authorities, but the OSS says, "If the water looks appealing, it probably is clean and clear enough for you, and if it looks unappealing, scummy or cloudy, then your instinct to stay out is a good one." If the cleanliness of the water in the lake did not cause any health problems previously, there is no reason that pregnancy should increase the risks. The potential to become infected through cuts and grazes also can be minimized by avoiding areas of water in which you are unable to see what's beneath you.
Play It Safe
Keep in mind that your body shape will continue to change during your pregnancy, so entering and exiting the lake might become more difficult. Take care to ensure that you minimize the risks of slipping or falling by identifying a safe exit point before you get into the water. Even in a lake that is familiar to you, each new stage of your pregnancy might change your ability to cope with natural hazards such as underwater currents, so it always is advisable to swim in the company of others when in open water.