First-born girls are not as quick to cast off their virginity as their younger sisters, new research shows.

A human biology conference in Australia was told that the order in which children are born into a family affects their reproductive behavior later in life.

Previous research has shown that the oldest children tend to be more confident and family oriented while the middle children have the toughest time.

Now it seems there are differences when it comes to reproductive strategies, including the age at which siblings have their first sexual experiences and first pregnancies.

Middle children stand out in the survey results, with females 2.6 times more likely to get pregnant at any age. Middle male children have sex for the first time at a younger age than their brothers.

But middle children of both sexes tend to have fewer children of their own and youngest female siblings tend to be younger when they have sex for the first time.

Researcher Fritha Milne, from the University of Western Australia, said the oldest children tend to align themselves with the "parental status quo," which partly explains why they're more conservative in their sexual choices.

Click here to read more on this story from news.com.au.