Since the birth of the triplets in 1997, the couple, from Thrybergh in South Yorkshire, have had a fourth child, conceived naturally and unexpectedly.

BBC: Triplets mother wins IVF case

In a study of more than 13, 000 UK children, five-years-olds were about twice as likely to have asthma if they were not conceived naturally.

BBC: Fertility treatment 'asthma link'

Preston, who hasn't said whether she conceived naturally or with the help of donor eggs or any other form of technology, joins a list of celebrities who became pregnant in their mid- to late 40s.

CNN: Pregnant at 47: Can I do that?

There is evidence, however, that IVF babies are at greater risk of birth defects than naturally conceived babies.

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