[The Saucepan Man]

Perhaps Elvish females remains fertile for only a short (in Elvish terms) period (no pun intended), say 150-200 years or so. This would explain why their children are born relatively early on in their marriages.

Possibly, as silqeleni suggested, the onset of fertility is triggered by impending marriage. Does anyone know of Elvish children born out of wedlock?

Also, I think someone indicated that most Elvish children tend to be born in Spring. Perhaps the females are only fertile once per year (which would certainly go some ways towards relieving the burden of menstruating for some 200 years). Since their gestation period is a year, this would tie in with our own ideas about Spring being the season of fertility (Rites of Spring, new born lambs and all that).

Pure speculation, I know, but it seems to make sense to me.