THURSDAY, Aug. 3, 2006 (HealthDay News) -- The usual examination that doctors perform to determine the position of a baby in a mother's womb in late pregnancy isn't sensitive enough and misses breech and other abnormal fetal positions, a new Australian study suggests.
Knowing the position of a baby in the womb is important, because if the fetus isn't in the normal head-down position -- called cephalic presentation -- vaginal delivery may be difficult or impossible. A non-cephalic presentation discovered after the start of labor can increase complications and even result in the death of mother or child.
"Most babies are born from 37 weeks of pregnancy, and are born in a head-down position," said lead researcher Natasha Nassar, a research associate at the Centre for Perinatal Health Services Research in the School of Public Health at the University of Sydney.
But breech presentation is when the baby "presents" with the buttocks or feet -- rather than the head -- first. "The rate of breech presentation among singleton births at term is 3 to 4 percent per annum. Transverse or oblique lie is when the baby is positioned horizontally in the uterus across the birth canal. This is relatively rare and occurs in one in 2,500 births, and requires a Caesarean section delivery," Nassar explained.
The fetal presentation is usually diagnosed by an obstetrician feeling the mother's abdomen, but the accuracy of this exam in late pregnancy is not known.
For the study, the researchers identified 1,633 women pregnant with one child at 35 to 37 weeks' gestation. Each woman had a clinical exam to assess the position of the baby. This was followed by an ultrasound scan to confirm the diagnosis.
The clinical exam found 70 percent of breech presentations, the researchers reported. More diagnoses were correct when women had had a previous pregnancy and lower body mass index.
Applying this percentage to a general population of 1,000 pregnant women, a clinical exam would identify 101 women with breech presentation, but 56 would actually have a breech presentation, and 24 women with one would be missed, according to the report.
The study was expected to be published in the Aug. 5 edition of the British Medical Journal.
If a baby is not diagnosed as being breech until after the mother's labor has started, then an emergency Caesarean section is necessary, Nassar said. "Emergency Caesarean section is associated with surgical complications and has the least favorable outcomes in terms of maternal morbidity and mortality and limits choices for women in their subsequent delivery," she said.
"Knowing that a baby is breech before labor begins means that the mother and her doctor have a choice about delivery options," Nassar said. "They can consider the option of an external cephalic version (ECV) to turn the baby or plan for an elective Caesarean section," she said.
The accuracy of a clinical examination to assess fetal presentation is inadequate for detection and timely management of a potential breech birth, Nassar said. "For particularly overweight or obese women, an ultrasound in late pregnancy would be beneficial for these difficult-to-assess cases," she said.
One expert thinks that ultrasound should be done in late pregnancy to accurately determine the infant's position in the womb.
"This is an important study in light of recent compelling evidence that, at term, fetuses in a breech presentation do better if delivered by planned Caesarean delivery prior to labor," said Dr. Charles J. Lockwood, chairman of Yale University School of Medicine's Department of Obstetrics, Gynecology and Reproductive Sciences.
For women whose fetuses are in a breech presentation, the best alternative to a Caesarean section is an ECV, in which the baby is turned to a head-first position by an obstetrician manipulating the fetus through the mother's belly, Lockwood explained.
"ECV is best done at around 37 weeks," he said. "Thus, knowledge of the breech presentation is key. We have long known that obstetrical caregivers are better able to detect breech presentations in thin women and those with multiple prior babies. However, given population trends toward obesity and fewer children, it is not surprising that the study suggests we do a poor job of detecting breech babies by physical examination -- missing 30 percent of cases."
Although the study was done in Australia, Lockwood suspects that similar results would be found in the United States. "The bottom line is that obstetrical-care providers who have office ultrasound machines would be wise to double-check fetal position at 35 to 37 weeks with a limited ultrasound examination," he advised.
The American Academy of Family Physicians can tell you more about breech births.