The team studied the levels of chemicals in the mothers during pregnancy and in the children at nine years old.

They then tracked onset of puberty and found exposure to certain chemicals was linked to the early onset of puberty in girls but no evidence that chemical exposure affected boys.

Lead researcher Dr Kim Harley, associate professor in public health at the University of California, said they found two chemicals were particularly significant.

“Mothers who had higher levels of two chemicals in their bodies during pregnancy – diethyl phthalate, which is used in fragrance, and triclosan, which is an antibacterial agent in certain soaps and toothpaste – had daughters who entered puberty earlier,” she said.