United States health officials say the number of twin births nationwide has dropped to their lowest level in 10 years.The U.S. Centers for Disease Control and Prevention, CDC, reported the decrease.Twin births nationwide began to increase about 40 years ago, CDC records show.From 1980 until 2014, the total number rose 79 percent.But from 2014 to 2018, the twin birth rates fell 4 percent.Last year, there were 32.6 twins for every 1,000 total births, the CDC reported.While the latest drop was not huge, officials said they consider it a good sign.Pregnancy with more than one child raises the risks of complications and death for both the mother and the baby.Dmitry Kissin is a doctor who works with the CDC's Division of Reproductive Health.He told The Associated Press that many people do not realize there can be serious risks with twin births."In the media," he said, "there are often stories about very cute twins andtalking about how great to have two babies at the same time."Such stories ignore the health risks, Kissin added.In 2018, about 124,000 babies born in the United States were twins. That represents about one in every 31 births.Health experts say there is no single explanation for the falling twin birth rates.However, Joyce Martin, the lead writer of the report, suggested one likely reason.She noted that the falling rates were only among white women and in women 30 years of age or older.Women from these groups are the biggest users of in vitro fertility treatments.In vitro fertilization is when doctors combine male and female cells to create an embryo,which is later implanted in a woman's uterus.This technique results in about 15 percent of all births involving more than one baby.In the past, it was common for fertility experts to implant twoor more embryos to improve the chances of a successful birth.This greatly increased the woman's chances of having more than one baby.In recent years, however, the method has changed.Fertility specialists now grow embryos longer in the lab and carry out more tests for genetic problems.They then choose the best single embryo to implant.The CDC's Dmitry Kissin says that today, about two-thirds of in vitro fertility treatments involve just one embryo.Aaron Styer is an infertility treatment specialist in Chestnut Hill, Massachusetts.He told The AP the success rate for one embryorose from about 20 percent in the 1980s to around 50 percent today.Styer said that health insurance companies and medical organizationshave increasingly been pushing doctors to implant just a single embryo.More and more patients are also accepting the methodto improve their chances of a successful pregnancy, he added.