In osteoporosis there is reduced bone mineral density which predisposes to increased risk of bone fracture.
Osteoporosis leading to fracture can occur during pregnancy, though it is very uncommon.
There is likely to be a background condition such as such as coeliac disease, osteogenesis imperfecta or a history of anorexia nervosa. In some pregnant women there is no identifiable cause. When there is no background cause it is called “pregnancy-associated osteoporosis”. In these cases it is not known whether pregnancy causes the osteoporosis or merely coincides with it.
Smith R, Phillips AJ in Osteoporosis during pregnancy and its management. Scand J Rheumatol Suppl. 1998;107:66-7 report “Typically the loss of bone leads to vertebral fracture with loss of height or pain in the hips also sometimes with fracture. Symptoms most often begin in the third trimester of the first pregnancy and improve after delivery; they do not usually recur in subsequent pregnancies. The cause is unknown and there is no specific treatment; follow up bone density measurements show that the osteoporosis slowly improves postpartum. Recent research in non-osteoporotic women shows that breastfeeding maintains a low bone density; it is therefore contraindicated in pregnancy-associated osteoporosis.”