Many have told me I'm too young to run these distances, to wait until I'm older.

CNN: Dad's death fuels teen's passion to race

Or would it make more sense to wait until your kids are a little older and you have more time to dive into Plan B?

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Professor Leridon then looked at women's chances of becoming pregnant through assisted reproductive techniques - assuming women would wait less time to start IVF the older they were.

BBC: The researchers evaluated women's chances of conceiving