A federal study of 15 million births indicates that 29 types of birth defects are becoming more common, especially defects that affect the heart and other parts of the cardiovascular system.

Two kinds of birth defects appear to be decreasing in incidence while rates for seven others have remained stable, the study said.One type of birth defect affecting the valves of the heart called pulmonary valve atresia and stenosis registered a 29 percent jump in incidence, the largest increase of the defects studied.

The study was a part of the federal Centers for Disease Control's birth defects monitoring program, which contains records of 15 million births that occurred in 1,200 mid-sized community hospitals. The study results were published in one of the CDC's surveillance summaries.

The birth defect program analyzes 161 categories of defects to identify increases or decreases. Only 38 defects were included in the study, which compared the number of defects found between 1979 and 1980 with those identified between 1986 and 1987. The defects were selected in part because "they are of sufficient severity to be of public health concern," the CDC said.

One of the more publicized types of birth defects studied, spina bifida, declined by 2 percent, the CDC said.

The decrease in spina bifida and another defect called anencephalus - in which the baby is born missing most of the brain - may have resulted from more prenatal diagnosis and maternal lifestyle changes, "such as better nutrition, which may decrease the risk of these defects," the CDC said.

However, fetal alcohol syndrome, a condition caused by alcohol consumption by a woman during pregnancy, increased by 10.6 percent, the CDC said.

"Almost half of the malformations with increasing trends were cardiac defects," the CDC study said.

"Although the increases in these cardiac defects are dramatic, the increased rates may be the result of improvements in health care and advances in diagnostic techniques," the agency said.