Two serious birth defects in the U. S. are on the decline

May 10 2004

Neural tube defects (NTDs) are serious birth defects of the spine (e.g., spina bifida) and the brain (e.g., anencephaly) that occur during early pregnancy, often before a woman knows she is pregnant; 50%--70% of these defects can be prevented if a woman consumes sufficient folic acid daily before conception and throughout the first trimester of her pregnancy. In 1992, to reduce the number of cases of spina bifida and other NTDs, the U.S. Public Health Service (USPHS) recommended that all women capable of becoming pregnant consume 400 µg of folic acid daily. Three approaches to increase folic acid consumption were cited:

1. improve dietary habits,
2. fortify foods with folic acid, and
3. use dietary supplements containing folic acid.

Mandatory fortification of cereal grain products went into effect in January 1998; during October 1998--December 1999, the reported prevalence of spina bifida declined 31%, and the prevalence of anencephaly declined 16% (2). Other studies have indicated similar trends. To update the estimated numbers of NTD-affected pregnancies and births, CDC recently analyzed data from 23 population-based surveillance systems that include prenatal ascertainment of these birth defects.

This report summarizes the results of that analysis, which indicate that the estimated number of NTD-affected pregnancies in the United States declined from 4,000 in 1995--1996 to 3,000 in 1999--2000. This decline in NTD-affected pregnancies highlights the partial success of the U.S. folic acid fortification program as a public health strategy. To reduce further the number of NTD-affected pregnancies, all women capable of becoming pregnant should follow the USPHS recommendation and consume 400 µg of folic acid every day.

The numbers of annual NTD-affected birth defects were calculated from a 24-month prefortification period (1995--1996) and a 24-month postfortification period (1999--2000). To calculate the number of NTD-affected pregnancies (including live births, stillbirths, fetal deaths, and elective terminations), CDC estimated prevalence for spina bifida and anencephaly obtained from eight population-based surveillance systems that collect data systematically from sources that perform diagnostic prenatal ultrasounds as part of their surveillance programs.

The numbers of spina bifida--affected pregnancies and anencephaly-affected pregnancies were calculated separately and then added together to provide an estimated total of NTD-affected pregnancies. Because the eight systems did not separate prenatally ascertained pregnancies from births, fetal deaths, and elective terminations, the remaining 15 population-based birth defects surveillance systems, which do not collect prenatally ascertained cases, were used to estimate the number of live births, stillbirths, and fetal deaths (occurring at >20 weeks' gestation) affected by NTDs. Previously published research on the ascertainment of NTD-affected pregnancies indicated that 9%--42% of such pregnancies were diagnosed prenatally.

The number of live births used as the denominator for calculating the prevalence estimates is published by CDC's National Center for Health Statistics. In 1999, approximately 6.9 million pregnancies occurred in the United States, resulting in 4.0 million births, 1.3 million induced abortions, and 1.0 million fetal deaths. In one study, 87% of fetal deaths occurred during the embryonic period, when a diagnosis of an NTD would rarely be made or included in existing birth defect surveillance systems. Adjusting the denominator for those remaining (13%) fetal losses occurring at 14--20 weeks only slightly modified the prevalence and numbers of cases determined in the calculations.

On the basis of data from the eight systems with prenatal ascertainment, an estimated 2,490 spina bifida--affected pregnancies and 1,640 anencephaly-affected pregnancies occurred annually before fortification of food with folic acid. The total annual average number of NTD-affected pregnancies was 4,130. After fortification, an estimated 1,640 spina bifida--affected pregnancies and 1,380 anencephaly-affected pregnancies occurred, for an annual average of 3,020 NTD-affected pregnancies (a 27% decline). On the basis of data from the 15 systems without prenatal ascertainment, an estimated 1,980 spina bifida--affected births and 970 anencephaly-affected births occurred annually before fortification, for an annual average total of 2,950 NTD-affected live births, stillbirths, and fetal deaths at >20 weeks' gestation. After fortification, an estimated 1,340 births affected by spina bifida and 840 births affected by anencephaly occurred, for a total of 2,180 NTD-affected live births and stillbirths per year (a 26% decline). The difference between the number of cases reported from systems with and without prenatal ascertainment suggests that an estimated 1,180 fetal deaths (occurring at <20 weeks) or elective terminations occurred before fortification, compared with 840 after fortification.

Reported by: P Mersereau, MN, K Kilker, Battelle Centers for Public Health Research and Evaluation; H Carter, MPH, E Fassett, MS, Assoc for Teachers of Preventive Medicine, Atlanta, Georgia. J Williams, MSN, A Flores, MPH, C Prue, PhD, L Williams, MPH, C Mai, MPH, J Mulinare, MD, Div of Birth Defects and Developmental Disabilities, National Center on Birth Defects and Developmental Disabilities, CDC.