Can a DIY fertility test help you plan when to have a baby?

My sisters, aged 27 and 30, are seated at their computers poring over the slick websites of companies promising to reveal secrets to them about their fertility. “Get insight into how your fertility is tracking relative to your age,” promises one. “Get the tools you need to have more control over your fertility,” says a second. “Gauge how long you have left to conceive,” says a third. The tests, which look at the levels of one or more female hormones in the blood, style themselves as easy to order and are less than what one would pay in a fertility clinic. “The information seems relatively cheap and readily available, so why not find out?” says my older sister. “I just assumed I wouldn’t have any fertility issues,” says the younger. “I realise after looking at these websites I probably shouldn’t assume this.”

These companies are the latest outgrowth of the growing global market in fertility services. Fuelled by women delaying childbirth longer, it includes IVF and egg freezing and is expected to generate $21bn (£15.5bn) in revenue globally by 2020....