Rates of births and abortions in the United States again declined in the most recent years for which data is available, as women experience fewer pregnancies, according to analyses released Wednesday by the Centers for Disease Control and Prevention.

The birthrate reached its lowest point in more than three decades, with 3,791,712 births registered in 2018. That total is 2 percent below the number reported in 2017, marking the fourth year in a row that births have declined.

The CDC received reports of 623,471 abortions in 2016, down 2 percent from the prior year. The figure represents the lowest number of abortions reported since the Supreme Court decided Roe v. Wade in 1973, as well as the lowest rate.

Decreases in births and abortions over the past several years have been pegged to fewer millennials having children than previous generations and a decline in teenage pregnancy. The latter trend is probably due to teens having less sex and better access to effective and long-lasting forms of contraception, experts have said.

Birthrates have been trending downward overall since 2005, sparking concern about potential economic and cultural ramifications. Keeping the number of births within a certain range, called the “replacement level,” ensures the population level will remain stable. A low birthrate runs the risk that the country will not be able to replace the workforce and have enough tax revenue, while a high birthrate can cause shortages of resources.

The total fertility rate, which measures lifetime fertility, in 2018 was 1,729.5 births per 1,000 women, which is below the replacement level of about 2,100 births. The country’s fertility rate has mostly been below replacement since 1971.

The rate of teenage pregnancy continued to decrease last year, with 17.4 births per 1,000 people — down 7 percent from 2017 and down 72 percent from the 1991 high. Women ages 20 through 34 also had fewer children, while birthrates for women 35 and over increased slightly.

The Guttmacher Institute, a research and policy organization that advocates for abortion rights, said in a statement that the overall decrease in births signals that fewer pregnancies were the main cause of the decline in abortions.

The 394 new restrictions on abortion that 32 states enacted between 2011 and 2017 did not primarily cause the decline, according to Guttmacher. Several conservative states this year implemented new restrictions in the hope of elevating a challenge to the conservative-leaning Supreme Court. Lower courts so far have repeatedly blocked the new state laws.

In 2016, there were 11.6 abortions per 1,000 women, compared with 11.8 abortions per 1,000 women the previous year. Women in their 20s accounted for the majority of abortions, while teenagers composed the age group with the largest drop in abortion rates between 2007 and 2016.

Almost two-thirds of abortions in 2016 were performed at or before eight weeks of gestation, and 91 percent were performed at or before 13 weeks. Roughly 60 percent were surgical abortions before or at 13 weeks, about 9 percent were surgical abortions after 13 weeks and around 28 percent were abortions induced by medication at or before eight weeks.

Read more: