While clomiphene increases the likelihood of pregnancy with one or two fetuses, gonadotropins increase the risk of conceiving twins or more.

However, Zhang falls in love with the maid Yuenu instead, and conceives twins with her.

Even without assisted reproduction, older women are more likely to conceive fraternal twins because, as women age, their ovarian follicles tend to release more than one egg at a time, Drs. Fong and McGovern explained.

Older women are naturally more likely than those under 30 to conceive twins or triplets, and they are more inclined to seek fertility treatments that result in multiple births.

'I had conceived twins and the other was still safely embedded in my womb,' she says.

Women over 35 contributed disproportionately to the increase; they are naturally more likely to conceive fraternal twins and are also more likely to require the fertility treatments that tend to yield multiple babies.

(Older mothers, who are more likely to conceive twins, and fertility treatments are two factors.)

One of the Relativists had conceived and bore twins on the voyage.

Firebird conceives male twins (later named Kinnor, after an instrument called a kinnora, and Kiel, for a bird of paradise on Netaia) and gives birth to them.