One of the reasons the question gets asked a lot is that some people have strong ideas about only children or how close in age kids should be. Many think it’s best to get the sleepless nights out of the way all at once; others say bigger age gaps can be easier. Some even suggest people should time their pregnancy with their friends'. Generally, though, life doesn’t work like that. One US study revealed that, in 1995, 1.8 million women suffered from secondary infertility (the inability to conceive or carry to term a second child); in 2006, it was 3.3 million . In 2010 it accounted for six out of 10 infertility cases. There are also around three million single parents in the UK , some of whom would like more children but haven't yet met the right partner.