A woman’s fertility peaks at the age of 24. There is a gradual decrease in fertility after the age of 30, so a couple may take longer to conceive, or have to face problems of sub-fertility. More pregnant women in this age group will have undergone fertility treatment.
It is not impossible or bad for a woman to fall pregnant if she is in her late thirties or forties, but one should take into account that there could be more complications. These complications are usually the result of other diseases that are more common in older people like hypertension and diabetes. There is also a greater chance of chromosomal abnormalities. Fortunately chromosomal abnormalities can be detected in the first half of pregnancy with a screening test (triple test or nuchal translucency followed by a definitive test: amniocentesis and karyotyping).
(Ilse Pauw, Health24)