The placenta is a curved structure and is too difficult to measure. If they had to measure the placental volume, they would need a very expensive machine, specialised training and more time.

With the help of his father Merwin Kliman, a mathematician and electrical engineer, Harvey Kliman developed an equation that used the maximal width, height and thickness of the placenta.

'In this study, we showed that the equation predicted the actual placental weight with an accuracy of up to 89 percent,' said Kliman.

'The method works best during the second and early third trimester, just when routine ultrasound screening is done on many women in the US.'

These results were published in the Monday issue of the American Journal of Perinatology.