Swimming for Two

OSS survey on pregnancy and outdoor swimming tests positive

Hannah Maia

With more and more women continuing to pursue their chosen sports well into the later stages of pregnancy, should outdoor swimming be any different? We discuss the results of a recent survey with OSS medic Dr Mark Harper and consultant obstetrician and gynaecologist Dr Katherine Fraser 

In 2018, The Outdoor Swimming Society helped carry out a survey with women who had continued to swim outdoors during pregnancy. Led by resident OSS medical advisor and consultant anaesthetist at Brighton and Sussex University Hospitals, Dr Mark Harper, the survey asked for details of the water temperature, any adaptation of swimming habits and any effects on the health of the mother and baby. 

The key question pregnant swimmers ask the OSS all the time is: is it safe? Will swimming outdoors have a negative effect on either of the mother’s of the baby’s health – either as a result of pollution, or cold? This survey had no way of giving a definitive answer on these questions.

“It would be very difficult to conduct a clinical trial on cold water swimming during pregnancy, so asking women about their experiences may provide the best evidence we can get,” says Dr Harper. 

Hannah Maia


Fifteen of the 25 women who responded swam outdoors until term. Few respondents noted the temperature of the water and the frequency of swims varied.

“Of the 25 responses, the majority were very positive about continuing to swim outdoors while pregnant,’ says Dr Harper. ‘This is very reassuring. It is, however, important to be aware that during pregnancy your body is pushed towards its physiological limits and doesn’t have as much reserve as usual.”

The positives reported in the feedback included:

  • 16 of the 25 respondents reported ‘No negative effects or health complications’
  •  Several highlighted the ‘good feeling of weightlessness’  and ‘buoyancy’ that swimming allows.
  • Several noted that it was sometimes the only time they felt free – physically and emotionally. This was partly due to its non-weight bearing nature, but also due to the therapeutic qualities of water itself and the natural highs that are released when the water temperatures drop.
  • Some of the respondents swam right up to their due date enjoying the ability to move ‘normally’ and finding that ‘a lot of the uncomfortable feelings disappeared in the water’.  

Other specific benefits were cited in a free comment box: ‘a better quality of sleep’; ‘my muscles feel like they’ve had a good work out, but also relaxed’; and ‘I could tolerate colder temperatures for much longer’

The negatives reported in the feedback included:

  • feeling more tired more quickly; 
  • loss of buoyancy causing sinking;
  • getting out of breath more quickly;
  • extremities feeling colder than usual;
  • feeling anxious about whether it was safe to swim outdoors;
  • not being able to bend over to take off neoprene booties if worn (!)

Respondents also mentioned that they made minor adjustments to their routine: for example, cutting back on distance or time in the water, or choosing their entry and exit points more carefully as they didn’t feel quite as nimble in the later stages. Many wanted to avoid trip hazards and felt a need to get into the water more slowly than they normally would.  

The clinicians involved believe the survey adds to anecdotal evidence that swimming during pregnancy does not cause ill effects for some women – and that there are women out there who have done short winter swims just before giving birth. (The survey did not include outdoor swimmers who didn’t swim outdoors during pregnancy for whatever reason).


Dr Katherine Fraser is an outdoor swimmer and consultant in obstetrics and gynaecology at Princess Royal Hospital (Brighton and Sussex University Hospitals) and Brighton and Sussex Medical School. Like Dr Harper, she is regularly asked whether it is safe to continue swimming outdoors during pregnancy. As there is little research or evidence, her advice is based on well-evidenced physiological changes during pregnancy that may be further exaggerated or impacted by cold water exposure.

“What is interesting is that women have complete different concerns than I would as a clinician,” she says. “For example, I was concerned about feeling faint, whereas patients were concerned about water quality.

“The pregnant woman has a higher cardiac output – with an increased heart rate and a lower blood pressure. In simple terms, the heart is already working harder than usual and so this may lead to lower exercise tolerance. Cold water may further impact this increased heart rate. Lower blood pressure in pregnancy is caused by dilatation of the peripheral blood vessels and can cause fainting and dizziness generally in pregnancy.

“Conversely, it may be expected that cold water may in fact cause vasoconstriction [constriction of the blood vessels] and increase the blood pressure a little. Ultimately, the core vessels are not usually affected by cold water and so I would not expect the placental blood flow to be affected or reduced. When swimming with my pregnant friend she always commented that she felt warmer during her swims than she had when non-pregnant.

“Controlling your breathing may be more difficult. As pregnancy progresses and the uterus grows, this heightens the diaphragm causing hyper-ventiliation (and, so, a higher baseline breathing rate) – this may be further impacted during entry into cold water and lead to panic. Muscular cramps – e.g. in the calf can also be more problematic in pregnancy and so pregnant women should be mindful of this. Cramps can usually be avoided with good hydration and eating pre- and post swim.”

“Regarding quality of water, your baby is safely enclosed within the amniotic sac and, as with swimming in a pool, whilst this sac is intact one should be fine to enter a lake or sea that would have been deemed safe to swim in outside of pregnancy. In lakes or rivers where wildlife contamination likely, particularly geese, rats, nearby cattle, heavy recent rainfall etc I would suggest caution on swimming and certainly ingesting any water. Weil’s disease (leptospirosis) crosses the placenta and can cause severe outcomes for fetus and mother. Also, nobody wants gastroenteritis when pregnant!”

“Finally, any woman who has required a cervical procedure in pregnancy (for example, a stitch) or whose membranes are not intact should not be swimming at all. I would not recommend open water swimming for any woman who has developed abnormally raised blood pressure or pre-eclampsia in pregnancy as these illnesses can ultimately lead to seizures.”

Kate Rew
Hannah Maia
Sara Barnes & Beth Pearson