The placenta supplies the foetus with essential nutrients
A measurement to spot small placentas could act as an early warning system and potentially stop babies dying in the womb, a study suggests.
Yale University researchers say they have developed an equation to work out the volume of the placenta with a high degree of accuracy.
They hope their measurement could spot problems with the organ, which nourishes the growing foetus.
The paper appears in the American Journal of Perinatology.
Very small placentas have been associated with foetal death, although many healthy babies are born of below-average size placentas and, conversely, sickly infants from larger ones.
Doing the maths
Harvey Kliman from Yale's department of obstetrics said he was inspired to develop an accurate means of measuring the organ after learning it was difficult to gauge on ultrasound screens.
The sum involves estimates of the maximum width, height and thickness of the placenta to produce the Estimated Placenta Volume (EPV), and is reported to predict its actual volume by up to 89%.
"The method works best during the second and early third trimesters," said Dr Kliman.
"I hope that the EPV test becomes routine for pregnant women."
At present there is nothing that can be done to improve or increase the size of placenta, but once it is noted a pregnancy could be monitored more carefully.
Patrick O'Brien, of the Royal College of Obstetricians and Gynaecologists, said the proposition had potential.
"It would be most useful if the calculation could be made earlier on in the pregnancy - later on we have a good idea of the health of the baby just by looking at it.
"I would like to see this as a study carried out on a large group of women to see if it does flag up problems at the outset. It certainly looks interesting."