She should think about spending some time in the sun, the study suggests
Children who are born in late summer or early autumn are often taller and stronger than peers born in spring and winter, a large study suggests.
The results from the Children of the 90s project - which involved 7,000 youngsters - says the reason may lie in their mothers' exposure to the sun.
The body makes Vitamin D, crucial for bone-building, from sunlight.
The Bristol University study suggests that this process may even occur in babies while still in the womb.
By the age of 10, those children born in the summer and autumn months were on average half a centimetre taller and had nearly 13 cm sq of extra bone area than those born in the winter months.
"Wider bones are thought to be stronger and less prone to breaking as a result of osteoporosis in later life, so anything that affects early bone development is significant," said Professor Jon Tobias, one of the researchers.
Mothers entering the late stages of pregnancy in the summer can attain the necessary vitamin D levels by walking around outside or even sunbathing, the researchers suggested.
People should not panic about skin cancer as a result of controlled exposure, as some sun was much better than none, they added.
And if there was not much sun to be seen, "women might consider talking to their doctor about taking Vitamin D supplements, particularly if their babies are due between November and May," said Professor Tobias.
In winter months at latitudes of 52 degrees north (above Birmingham), there is no ultraviolet light of the appropriate wavelength for the body to make vitamin D in the skin, research shows.
The Arthritis Research Campaign is currently running a trial to establish whether giving vitamin D to pregnant women increases the bone density of their babies at birth and in childhood and reduces the risk of developing osteoporosis in later life.
"Although most people in the UK can can get the essential nutrients they need from their diet, and don't need to take extra supplements, the exception is vitamin D," a spokeswoman for the charity said.
"Because of a lack of sunshine in the UK in winter many Brits are vitamin D deficient, with vitamin D deficiency extremely common in pregnant women, leading to their babies having weaker bones at birth."
A study last year also suggested that pregnant and nursing mothers take supplements to curtail an apparent resurgence of the bone disease rickets.