But this isn't the case for Olympian athlete Alysia Montano, who recently competed in an 800m race!
Alysia, who is a professional runner and five-times US national champ, ran the distance in an astonishing 2mins 32secs at Hornet Stadium in California.
The 28-year-old sportswoman is due to give birth in seven weeks and checked with doctors before deciding to participate in the race.
Quoted in the Daily Mail, Alysia said: "I've been running throughout my pregnancy and I felt really, really good during the whole process."
She added: " What I found out mostly was that exercising during pregnancy is acutally much better for the mum and the baby…I did all the things I normally do…I just happened to be pregnant."
We don't think we could even run this fast when we're not pregnant, so we're pretty impressed with Alysia's achievement!
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