LOS ANGELES (Reuters) -- The risk of miscarriage after undergoing chorionic villus sampling, or CVS, to detect birth defects is lower than previously thought and essentially carries the same risk than the more commonly used amniocentesis, according to new research.

The findings, although based on a fairly small sample of cases, are important because most women prefer prenatal screening as early as possible in pregnancy, and CVS is performed about six weeks earlier than amniocentesis.

Both procedures carry a small risk of miscarriage, but the study found that the risk attributable to CVS is the same as the risk of 1 in 370 seen with amniocentesis when adjusting for the earlier gestational age of the CVS procedure.

Amniocentesis, which involves inserting a needle through the abdomen into the uterus to withdraw amniotic fluid, is performed early in the second trimester at 16 to 20 weeks of pregnancy.

CVS involves collecting a small sample of placenta cells from the lining of the uterus either by inserting a slender needle through the abdomen or by inserting a thin plastic tube through the vagina into the placenta. It is done in the first trimester, between 10 and 12 weeks gestation.

Based on earlier research, women have been counseled that the miscarriage risk from CVS is greater than with amniocentesis.

Researchers at the University of California at San Francisco studied data from 1983 to 2003 on nearly 10,000 CVS and 31,000 amniocentesis procedures performed at a single prenatal diagnostic center.

The study, published in the September issue of the journal Obstetrics & Gynecology, found that the overall pregnancy loss rate over the 20-year period was 3.12 percent for CVS and 0.83 percent for amniocentesis.

When the researchers controlled for gestational age, maternal age, and indication for the procedure, they found no difference between losses from CVS or amniocentesis.

And, when examined in five-year intervals, the pregnancy loss rate from CVS dropped to 1.93 percent in the last five-year period of the study.

They suggested that the miscarriage rate from CVS may have decreased as doctors became increasingly proficient at performing the procedure.