Previous research indicated that exposure to testosterone in the womb could be linked to differences in finger length.
The Duke and Duchess of Sussex are expecting a baby next spring, Kensington Palace announced.
The princess had two 12-inch metal rods inserted in her back in an eight-hour operation, which took place in 2002.
Almost 2 million people have taken Public Health England's free online heart age test.
Little Kit died from the cold sore virus less than two weeks after birth.
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Thomas Markle watched the wedding from California where he was recovering from heart surgery.
The pair have known each other since their teens.