In 1998, about one in every 500 births was of triplets or more, according to the CDC. This was problematic in more than one way. For example, multiple babies tend to be smaller, and may be quite premature. Thus they may spend extended periods in neonatal intensive care units — an extremely expensive undertaking. Further, a multiple pregnancy can be hard for the mother to carry to term.
One reason for the high rate of multiple births is that when women delay maternity until later ages they may have difficulty getting pregnant. In that case, many resort to some sort of medical assistance such as in vitro fertilization (IVF). In these procedures eggs and sperm are mixed in the lab, and resulting embryos are then implanted in the mother. To be sure of a pregnancy, it has been the practice for fertility doctors to implant more than one (sometimes several) eggs into the mother. Thus the possibility of multiple births is greater.
However, lately the number of such births — particularly of triplets and higher multiples — has declined. The agency reports that "[t]he number of triplet and higher-order births declined from 7,625 to 4,526 from 1998 to 2014, or from about 1 in every 515 births in 1998 to about 1 in every 880 births in 2014."
But the decline was not equal across all demographics: first, the data apply only to women over the age of 25 — especially over 45 — and were largest for non-Hispanic white women — 46 vs. 15 percent in Hispanic women. The rate didn't change for non-Hispanic black women.
These decreased rates were common in most states, while only two (Oklahoma and Louisiana) had increases in higher order births, and these increases were small.
The reasons for the decreases in the numbers and rates of higher order births are likely the practices of fertility clinics, which are now advised to not implant multiple embryos. According to the report, "the trend toward older age at childbearing has continued throughout the study period with the percentage of births to women aged 30 and over rising from 36% in 1998 to 44% in 2014. Changes in assisted reproductive technologies (ART) practices, such as reducing the number of embryos transferred in ART procedures, have been associated with the decline in higher-order multiple births."
Whatever the reasons, this decrease in higher order births is a positive occurrence, since such babies are less likely to survive their first year of life compared to singletons, and are also more likely to suffer long-term ill effects.