Count the woman's age first if you plan to wait a few years to have a child: the chance of conceiving naturally and the success rate of IVF both fall clearly around age 40

In plain words

The older the woman is, the harder it is to conceive naturally. In the early 40s, the chance of conceiving in any given month is only about half what it is in the early 30s. Once pregnant, the share of pregnancies that end in miscarriage also rises, to about one third at ages 40 to 44. IVF (试管婴儿) cannot make up for it either: at ages 43 and 44, only about 5% of embryo transfers result in a baby being born.