If you have always found doing sums a struggle, you might just be able to blame your mother.
Because research has linked a woman's hormone levels in pregnancy with her child's maths skills at age five.
Boys and girls whose mothers were very low in the hormone thyroxine were almost twice as likely to do badly in arithmetic tests, it found.
Thyroxine, which passes from mother to baby in the womb, is crucial for the development of the brain - but many expectant mothers have too little of it.
Researcher Martijn Finken studied almost 1,200 children from when they were in the womb until they started school.
He measured their mothers' thyroxine levels 12 weeks into pregnancy and compared the results with the children's scores in arithmetic and language tests at age five.
Those who were exposed to the lowest levels of thyroxine in the womb were 90 per cent more likely to end up in the bottom half of the class for maths.
This was true even after taking into account the child's family background and health at birth.
Interestingly, hormone levels were not linked to the children's scores on tests of vocabulary and ability with language.
Dr Finken said this may be because our language skills owe a lot to our upbringing.
Our maths ability, on the other hand, can be traced more directly back to brain development.
The researcher, a paediatric endocrinologist from the VU University Medical Center in Amsterdam, will continue to track the children's progress as they go through their school years.
Dr Finken said: "Whether these problems persist into adulthood remains to be seen. We will continue to follow these children to answer this next big question."
He added that it may be possible to deal with the problem simply by testing women's hormones early in pregnancy and giving thyroxine supplements to those who need them.
Doctors have tried this in the past with little success, but Dr Finken believes that is because the supplements were not given early enough. Women should take the tablets in the first four weeks of pregnancy, he said.
British expert Professor John Lazarus explained that during early pregnancy, an unborn baby is not yet able to produce its own thyroxine.
This leaves it dependent on the mother's supply - but studies suggest that up to two-thirds of mothers-to-be in Britain are low in the hormone.
Professor Lazarus, who is a former trustee of the British Thyroid Foundation, put the widespread problem down to a lack of iodine in the diet. Iodine, which is found in milk and fish, is the main ingredient of thyroxine.
He suggested that iodine should be added to salt to boost people's consumption - a move that has been taken in many other countries.
The professor, who is chairman of the UK Iodine Group, advises women to take iodine during pregnancy. However, it is not currently on the list of supplements the NHS recommends for mothers-to-be.
Professor Lazarus said it is safe for pregnant women to take 100 to 150 micrograms a day.
British doctors have previously warned that many teenage girls are dangerously low on iodine - putting the health of their future children at risk.
A 2011 study of 700 schoolgirls revealed almost 70 per cent were deficient in the mineral.
Researchers said young girls are not drinking enough milk.
The NHS says most people should get all the iodine they need by eating a varied diet.
- Daily Mail