The age at which a girl first starts her periods has dropped over the past century because of better health and nutrition, which has meant that young girls today are bigger than their grandmothers were when they were the same age.

Size, especially the amount of body fat, is one of the most important determinants of when periods start. Genetics also plays a part – girls who start their periods young will often find that their mother did too.

The usual age now for a girl to start her periods is between 12 and 13, but there are plenty of girls who start earlier. Nearly one in 100 has started by the time they're ten, and about one in 30 by the time they're 11. So your granddaughter isn't abnormal at all, and it isn't necessary to take her along to the doctor unless there's something in particular that's worrying either her or her mother.

Talk about the changes

Although the start of periods doesn't immediately mean that a girl is fertile (the first few periods are 'anovulatory', which means no egg is released) it does mark the beginning of puberty. So it's a good time to start to explain to girls about sex, pregnancy and the changes that puberty brings if they don't already know. Your granddaughter needs to understand what periods are all about, but this can be a lot for a nine year old to take in, so take it slowly, starting gently with simple explanations, and include an emphasis on loving relationships.

Take a gentle approach, and include an emphasis on loving relationships. There are plenty of good books available to help.

Help her to cope at school

Although the statistics mean that one in eight girls has started their periods while still at primary school, most primary schools aren't well equipped to deal with this and lack suitable sanitary facilities or education about periods.

Your granddaughter may find it quite traumatic to deal with her periods at school, especially if none of her friends have started yet.

You could encourage your daughter to ensure your granddaughter is well equipped to cope with periods at school, can discreetly obtain sanitary towels if she needs them and has someone such as her teacher or a school nurse to turn to if she has any problems or questions.

This article was last medically reviewed by Dr Trisha Macnair in August 2008