Paula Radcliffe has decided to continue running during her pregnancy.
Radcliffe will not compete at the highest level while she is pregnant
The world marathon champion will train at the same level, but she will not race in major competitions.
So how much exercise should you do while pregnant, and is there any risk to the baby?
Patrick O'Brien, a spokesman for the Royal College of Obstetricians and Gynaecologists, provides some answers.
It is not inevitable that athletes will become less competitive after they become pregnant, but most will notice a reduction in their performance.
It's not harmful to the baby or to the athlete in any way, but someone like Paula Radcliffe will probably find as she gets bigger and heavier that it becomes more difficult for her to exercise to her peak capacity.
So she will probably end up having to reduce the amount of exercise she does, just because she doesn't feel up to it.
She would be best to be under the care of an obstetrician with specialist knowledge in exercise in elite athletes during pregnancy, but she'll find that she'll have to limit her exercise a bit.
The duration of each session might need to be reduced a bit and she will probably be able to tolerate a bit less herself.
Her body will direct her as to how much stress she is able to put on her body.
IS IT SAFE?
Apart from the gain in weight, there is also an extra demand on your heart during pregnancy because your blood volume increases by 50% and your heart has to pump all of that to get extra nutrition to the baby.
That will limit the amount of exercise you can tolerate.
The general feeling is that exercise during pregnancy is good from a psychological point of view.
It gives you a sense of wellbeing, makes you feel more positive and helps you to sleep better.
There is also some evidence that women who exercise during pregnancy are less likely to feel stressed or depressed.
I looked after someone who ran the London Marathon the week before she had her baby and she was fantastic
For all women in pregnancy, the middle third is the easiest and that makes it the easiest time to exercise.
The first 12 weeks can be pretty rough, what with the feeling of sickness and lethargy, and towards the end it can be tough just because of the weight of the baby.
The way that the baby loses heat is to the mother, so if the mother gets too hot then the baby starts to find it difficult to lose heat.
So the advice we give to all pregnant women during pregnancy is to try and avoid getting too hot, whether it's just during exercise or swimming in a warm pool.
I looked after someone who ran the London Marathon the week before she had her baby and she was fantastic.
But people who are so fit tend to find labour easier and quicker.
If someone feels up to it, fine, but obviously they have to build up to it gradually and make sure they can cope with it.
After pregnancy, things are obviously easier because all that weight and stress is gone, and it only takes about six weeks for the body to more or less get back to normal.
The one consideration is breast feeding.
The woman needs extra nutrition for the baby, so it's important she doesn't exercise excessively afterwards, or if she is doing a lot of exercise, makes sure she ups her nutrition appropriately.
Psychologically, pregnancy can make people stronger, although a lot of women nowadays have an epidural during labour which makes it less painful.
But pregnancy and childbirth are quite demanding on the body, so going through that must make somebody stronger afterwards.