Các bé gái có đang có kinh nguyệt sớm hơn không? Và cha mẹ có thể làm gì để thay đổi điều đó?

Do you remember how old you were when you got your first period? And whether it was considered early, late or about average?

Some parents are reportedly overhauling lunchboxes, shampoos and doing all they can to attempt to delay their child’s periods, so they don’t “grow up too early”.

But are periods actually getting earlier? And is this anything to worry about? Let’s look at what the science says – and whether parents can actually impact the timing.

When do periods start and has this changed?

Girls begin puberty between the age of eight and 13 years. Breast development, known as thelarche, is the first sign. Menarche, the first menstrual period, follows 1.5 to three years later.

The evidence suggests girls are entering puberty a little earlier.

A global review found thelarche began, on average, about three months earlier per decade from 1977 (10.5 years) to 2013 (9.6 years) but this varied across the world.

In Australia, the average age of menarche is about 12.9, but it’s unclear how this has changed over time.

In the United States, the average age of menarche fell from 12.5 years among those born from 1950 to 1969, to 11.9 years among those born from 2000 to 2005.

But the researchers asked the women to remember when their periods began. This risks inaccurate recollection, or recall bias, if participants can’t remember their age when they got their first period.

The typical range for periods to start is nine to 15 years and depends on factors such as ethnicity.

What if puberty starts earlier?

Precocious puberty means breast or pubic hair development before age eight in girls, not simply an early period. About 3% of children show signs before age eight.

If your child has breast or pubic hair development or a first period before they’re eight, take them to your family doctor for a check up.

Only a small proportion of girls – around 0.2% – have an underlying medical issue causing precocious puberty that requires treatment.

How does puberty start?

Puberty is triggered by the hypothalamus, a region at the base of the brain, when it releases gonadotopin-relaseasing hormones (GnRH) . This causes the chain reaction of chemical messengers for puberty.

The hormones reach the pituitary gland in the brain, which triggers luteinising hormone (LH) and follicle-stimulating hormone (FSH) . These hormones stimulate the ovaries to produce oestrogen and progesterone.

Genetics, body weight, diet, physical activity, environmental factors and family circumstances may all influence when these hormones are triggered.

Genetics

Some 400 genes and genetic markers are associated with the timing of the first period, so we can see how variable this can be.

A study of around 370,000 Icelandic women found that of the factors that contribute to the variance in the age at menarche, genetic signals account for around 25%.

Another study estimated the age of menarche was half attributed to genes and half to other factors.

Body weight

In more economically advanced countries, trends toward earlier puberty have correlated with changes in body weight.

This is often attributed to excess calorie intakes, but also with reduced physical activity in children.

Higher body mass index, based on weight in relation to height, predicts earlier thelarche and menarche, while earlier puberty may promote later weight gain.

But while this looks like a neat cycle, shared biological and social factors may affect both, making cause and consequence difficult to separate. Early menarche may be a risk marker rather than a cause of obesity.

Diet and exercise

Studies have found nutritional deficiencies – of proteins, essential amino acids, calories and essential fatty acids, as well as some micronutrients such as iron and zinc – can delay the onset of puberty.

Girls who play a lot of sport or dance at a young age sometimes have delayed periods. This may be a result of low energy availability, which can delay triggering the hormones that start puberty.

Restricting food or encouraging excessive exercise can delay periods because the body lacks energy – this can be harmful.

Environmental chemicals

Endocrine-disrupting chemicals are associated with the disruption of hormones during puberty and puberty timing.

In Australia, many have been banned, phased out or limited. This includes polychlorinated biphenyls (PCBs) , pesticides such as dichlorodiphenyltrichloroethane (DDT) , bisphenol A (BPA) and certain phthalates.

Some endocrine-disrupting chemicals are still found in personal care products such as shampoo, as well as in plastic bottles, containers and elsewhere in the environment.

But an association between earlier puberty and endocrine-disrupting chemicals doesn’t mean swapping every plastic container or shampoo bottle will postpone a period. We’re all exposed to them to some degree.

Family circumstances

Finally, social and family circumstances may also play a role in determining the age of menarche, with studies linking father absence and earlier menarche.

But financial strain, maternal stress and family disruption are difficult to separate, so the link may be related to these other factors.

What does this all mean?

Parents can’t control when their child’s period starts. No particular “clean” diet or exercise regimen is proven to safely delay normal puberty. And when it comes to plastics, reducing exposure is reasonable but you can’t control this entirely.

Puberty isn’t something to fear. A girl’s first period is an opportunity for parents to help her understand her body, prepare for what’s coming, learn how to manage periods and let her know when to seek help for pain.

Getting a first period doesn’t mean a girl is now ready for dating, having sex, or “losing innocence” in general. It doesn’t mean she has to “grow up”, dress differently, or stop enjoying her childhood.

Menarche is just a biological process, preparing the body for maturity. It means the body is healthy and doing what it’s supposed to do. And happens at different times for everybody.

Jodie Avery receives funding from the National Health and Medical Research Council.

Shae Maple receives funding from an Australian government Research Training Program scholarship and a Healthy Development Adelaide and Channel 7 Children’s Research Foundation PhD Excellence Award.