For pregnant women without symptoms, and who have ultrasound evidence that their fetus is viability at the first prenatal visit, the risk of subsequent miscarriage is less than two per cent -- well below previously reported rates -- according to a large study conducted in Australia.

The overall risk of miscarriage once a woman knows she is pregnant is 12 to 15 per cent, the researchers note in the journal Obstetrics and Gynecology. Even after ultrasound has confirmed that the fetus is healthy, studies have shown that early miscarriage rates can be as high as 12 per cent.

However, no studies have exclusively investigated the miscarriage risk among women without symptoms, such as vaginal bleeding or lower abdominal pain, Dr. Stephen Tong, from Monash University in Victoria, and colleagues point out.

Tong's group conducted a study with 696 asymptomatic women carrying one fetus who attended their first prenatal visit between six and 11 weeks of pregnancy. Ultrasound exams documented fetal cardiac activity.

Eleven patients (1.6 per cent) had a miscarriage before 20 weeks. The risk was relatively high at 9.4 per cent among the women who were six weeks pregnancy, falling to 1.5 per cent among those who were eight-weeks pregnant, and less than 0.7 per cent from nine weeks onward.

The study was conducted in a regular prenatal clinic, "not a specialized early pregnancy clinic or an ultrasound department," Tong's group points out, and none of the attending physicians who performed the initial ultrasonography had any formal subspecialty qualifications.

"Consequently, we believe that our data could be extrapolated generally to health care providers provided that they are able to offer office ultrasonography to confirm viability at that visit," the team surmises.

Ford's group concludes that, for asymptomatic women who have their first prenatal visit between eight and 11 weeks of pregnancy and do not have pregnancy risk factors, "our data may be useful as a counseling tool to reassure such women at the end of their first prenatal visit that their risk of loss is already very low."