There are currently 53.6 million women aged 35 to 49 who are unable to conceive, sparking fresh concerns over a global fertility crisis.Researchers from Chongqing Medical University analysed health data from 204 countries between 1990 and 2023 to assess how the 'infertility burden' among women aged 35 to 49 has changed over time.They found it has increased steadily since 1990, rising from around 6,001 per 100,000 women in 1990 to 6,907 per 100,000 in 2023.Women aged up to 49 are often included in reproductive-age statistics because some are still menstruating and could theoretically conceive, although doctors stress that fertility falls sharply from the mid-30s and is usually very low by the late 40s.However, the fastest projected rise was seen among women at the younger end of the group, according to the figures published in The Lancet Obstetrics, Gynaecology, & Women's Health.The study divided the women into three age groups: 35 to 39, 40 to 44 and 45 to 49. Infertility was defined as being unable to achieve a clinical pregnancy after 12 months of regular unprotected sex.Using the data to forecast how current trends could shape the future, the researchers found that infertility is expected to rise across all three age brackets, but the steepest projected increase is among women aged 35 to 39.This does not mean women in their late 30s are more likely to be infertile than those in their 40s, as fertility still declines with age because of falling egg number and quality. There's been a surge in women aged 35 to 49 who are struggling to conceive naturally Instead, the finding suggests that the overall burden of infertility is growing fastest among women at the younger end of the 'advanced maternal age' group, likely reflecting the fact that more women are delaying motherhood into their late 30s as they focus on education and their careers.But, by the time they feel ready to start a family, their fertility may already have begun to wane.In the UK, the Office for National Statistics report on births for 2021 showed the average age of mothers in England and Wales was 30.9 years – the oldest on record.This was part of a steady rise since 1973, when the average age of mothers was just 26.4 years.The ONS figures also revealed that births among women over 40 were more than double those among teenagers. Just five decades ago, there were nine times as many teenage mothers as mothers over 40.The authors of the new study also say obesity and stress may be having an influence on fertility.Greater access to fertility treatment may also have increased both the number of women seeking help and the number of cases being diagnosed.The other two age groups, 40 to 44 and 45 to 49, are also expected to see significant increases, bringing the total number of women affected by infertility to 79.6 million by 2036.'The subsequent increase from the late 1990s onward coincided with rising female participation in education and the labour force, leading to widespread delays in childbearing and a consequent rise in age-related infertility,' wrote the study's lead author, Yuanyuan Du.