NEW YORK (Reuters Health) - Older women undergoing assisted reproduction are less likely than younger women to end up having a baby. However, a new study suggests that age is only a factor in certain aspects of infertility treatments--mattering at some points in the treatment but not at others.

For example, older women are less likely to have a fertilized egg develop into the ball of cells necessary for implantation into the uterus, according to recent study findings. However, for those whose embryos do reach this stage--known as a blastocyst--their rates of implantation and pregnancy are similar to their younger peers, researchers report.

Another problem is that older women--even those in their early 30s--have fewer eggs than younger women. And "the eggs they do (have) are of lesser quality than they were before," lead study author Dr. Bruce S. Shapiro, of the University of Nevada School of Medicine in Las Vegas, told Reuters Health.

If, however, the woman produces eggs in sufficient quantity and quality "then the chances of getting pregnant with in vitro fertilization is still high," he added.

Shapiro and his colleagues investigated the influence of patient age on embryo implantation in a study of 300 women aged 18 to 45 who were undergoing in vitro fertilization (IVF). In IVF, a woman first takes ovulation-stimulating drugs, then her eggs are collected and mixed with sperm in a laboratory culture dish. Once the fertilized eggs develop sufficiently, they are transferred back into the woman's uterus.

In the study, women in their early- or mid-20s produced about 20 eggs per cycle compared with 10 eggs per cycle for women in their early- or mid-40s. For every additional 2 years in age the women produced one less egg.

However, age made no difference when it came to successful fertilization of the eggs, the investigators reported in a recent issue of Fertility and Sterility.

Yet, over 90% of women in their 20s ended up with at least one blastocyst that could be transferred, compared with less than 70% of patients in their 40s.

But again, age made no difference when it came to pregnancy rate after transfer. The researchers note that for each transfer procedure, over 45% of women aged 41 or 42 years had a pregnancy, though it is not clear if the pregnancies ended in miscarriage or went to term. Overall, the pregnancy rate was 40% per ovulation-stimulating cycle in the youngest patients and 25% in those in their early 40s. There were no pregnancies in patients over 43, though the study included only a few attempts in women this age.

"In the current study, although pregnancy rates per stimulation declined with age, implantation rates and pregnancy rates per transfer remained remarkably steady across the age range studied," Shapiro and colleagues conclude.

SOURCE: Fertility and Sterility 2002;77:700-705.