More accurate screening and an increase in the number of younger mothers being screened for Down's syndrome may have forestalled an expected increase in the number of babies born with the condition in the United Kingdom, a study shows.
More accurate screening of moms in the U.K. meant fewer Down's syndrome births, according to new research released this week.
From 1989 to 2008 in the number confirmed prenatal or postnatal diagnoses of Down's syndrome in England and Wales increased by 71 percent -- from 1,075 to 1,843 -- according to Dr. Joan Morris and Dr. Eva Alberman of Barts and the London School of Medicine and Dentistry.
However, the number of babies born with Down's syndrome remained relatively steady despite an increase the age of mothers -- there were 752 in 1989 and 743 in 2008 -- because of terminations following prenatal diagnoses, they reported online in the British Medical Journal. The termination rate remained unchanged.
Dr. James Goldberg of San Francisco Perinatal Associates, past chair of the American College of Obstetricians and Gynecologists' Committee on Genetics, said he didn't know whether there was a similar trend in the United States or in other countries.
He said he believes the situation would be much the same here, although at least one small study has shown an increase in the number of live Down's syndrome births over time in women older than 35.
A spokesperson for ACOG noted that there are no national U.S. data on the number of women who receive prenatal screening for Down's syndrome or reliable data on the number babies born with the disorder.
Morris and Alberman examined data from the U.K.'s National Down Syndrome Cytogenetic Register, which includes about 93 percent of all diagnoses of Down's syndrome in England and Wales.
The database included information on diagnoses in live births and stillbirths, as well as antenatal diagnoses and subsequent terminations, fetal losses, and a small number babies brought to term.
Over the 20-year study period, the large increase in diagnoses occurred even though the overall number of births remained constant.
Starting in about the mid-1990s, the researchers said, screening for Down's syndrome has been improved through new methods and technologies.
Aside from the slightly different ages at which pregnant women were previously recommended for screening in the U.K. and U.S., Goldberg said, screening in the two countries is similar.
Now, in both countries, it is recommended that all pregnant women receive an offer of screening with a discussion of the pros and cons.
Without these improvements in screening, Morris and Alberman said, the number of babies born with Down's syndrome would have increased by an estimated 48 percent -- from 959 to 1,422 -- as a result of women waiting until later in life to have children.
A 40-year-old has about 16 times the risk of having a baby with Down's syndrome as a 25-year-old, they said. During the study period, the proportion of women younger than 37 who underwent screening increased from 3 percent to 43 percent.
The proportion of women 37 and older who underwent screening, on the other hand, remained constant at about 70 percent.
Morris and Alberman said additional studies to determine why more older women don't receive screening for Down's syndrome would be valuable.
"To ascertain whether the decision is an informed one and, if not, to address the lack of information, is important," they said.
During the study, the percentage of pregnant women who received diagnoses of Down's syndrome and went on to have the pregnancy terminated remained steady at about 92 percent.
Goldberg said that in California, data indicate that this proportion is closer to 60 percent.
But he noted that a large database of all Down's syndrome diagnoses does not exist in the U.S., and the figure is likely to vary widely throughout the country.
The ACOG spokesperson said that it's unclear how many terminations result from learning of a Down's syndrome diagnosis, because "[neither] the CDC nor anyone else collects data on the indications for pregnancy terminations."