July 10 (HealthDay News) -- Measuring the height of the uterus of a women pregnant with twins after in vitro fertilization (IVF) can help determine her risk of having the babies born prematurely, a new report says.

Doing this using ultrasound can help doctors and women decide how many embryos should be transferred in one IVF attempt, according to the research, presented Wednesday at the European Society of Human Reproduction & Embryology annual meeting, in Barcelona, Spain.

"Twin pregnancies account for between a quarter and a third of pregnancies obtained during IVF, and 8 percent of them are complicated by the babies being born extremely premature, leading to medical complications and sometimes fetal mortality," presenter Raphael Hirt, a fellow in the Division of Reproductive Medicine at the Hospital Antoine Bclre in Paris, said in a news release from the European Society. "For this reason, single embryo transfer is promoted as the best way of avoiding twin pregnancies, but, in some cases, this can alter the overall likelihood of pregnancy. An evaluation of a woman's individual risk of perinatal adverse outcomes from a twin pregnancy may help to select those women who have a lower risk of having twins born severely prematurely and who could consider a double embryo transfer if that is what they want."

The researchers had previously observed that women who already had children were less likely to give birth prematurely. They theorized that this was because the uterine cavity had been distended by the previous pregnancies.

Using transvaginal ultrasound (a process called hysterosonometry, or HSM), they measured uterus height of 79 women and followed them after having successful IVF treatment.

Women with the smallest uteri (less that 63 millimeters high) had a much greater chance of having babies born severely premature with an increased number of fetal deaths -- seven fetal deaths in this group compared with one in the women with medium-size uteri (between 64 and 70 millimeters high) and none among those with uterus height greater than 70 millimeters. Six deaths in the small uteri group were linked to prematurity, while the one death in the mid-sized group was not.

After adjusting for any confounding factors such as age, parity, previous premature delivery, the researchers found a clear association with HSM.

"Our results show that HSM is a reliable and noninvasive method for predicting twin-related severe prematurity and neonatal mortality, and it can be used before conception to help with objective decision-making about the number of embryos to transfer," Hirt said. "For women with an HSM measurement of less than 62mm, a single-embryo transfer is indicated, but in those with a longer uterine cavity, a double-embryo transfer can be considered if it is acceptable to the patients."

The researchers plan to use their new criteria going forward and see whether it results in decreased fetal mortality. They also plan to study whether a short uterine cavity affects single pregnancies to find whether it can identify which women may need intensive neonatal care.

SOURCE: European Society for Human Reproduction & Embryology, news release, July 8, 2008