TOKYO - Some 37.8 percent of women aged between 25 and 49 years in Japan have experienced or are experiencing infertility partly because more couples choose to have children later in life, a university study found Tuesday.

By comparison, the World Health Organization estimates 17.5 percent of the global adult population experience infertility, which is defined as the "failure to achieve a pregnancy after 12 months or more of regular unprotected sexual intercourse."

"One reason can be attributed to people wanting children later in life, as evidenced by the rise in the age of first-time mothers," said Eri Maeda, associate professor at Hokkaido University and head of the research group.

The average age of first-time mothers in Japan in 2024 was 31.0 years old, compared to 27.5 in 1995, among the higher bracket in the Organization for Economic Cooperation and Development countries.

"Infertility is also common among men. It's a mistake to consider it a 'woman's issue,'" Maeda said.

A total of 3,000 women were selected from the nationwide resident register listing all members of each household between January and March 2024, with the university receiving 1,200 valid answers and analyzing 853 responses of people who were married or had a male live-in partner.

The survey also found that just 8.3 percent of respondents were having sex more than once a week, compared to around 60 percent among married women in the United States in a study released in 2020.

"There is a possibility that the infrequency of sex is prolonging the time needed to become pregnant and raising infertility rates," Maeda said.

The study additionally listed long work hours, living environments and a culture that fails to prioritize one's sex life as factors that may affect fertility.

The results come as the number of children in Japan born via in-vitro fertilization treatment hit a record high in 2023 at around 85,000 in 2023, the equivalent to one in 8.6 people, according to the Japan Society of Obstetrics and Gynecology.

In April 2022, fertility treatments were expanded to public insurance, opening doors to a process that could otherwise be cost prohibitive, but there have also been calls to eliminate limits on ages and frequency.

A study conducted by the National Institute of Population and Social Security Research in 2021 also found that 39.2 percent of couples had expressed concerns about infertility.