Indeed. But in MORGOTH'S RING Tolkien explains how they actually lost a part of their strength or spirit in reproducing, so they would normally have at most only four or so children. Feanor and Nerdanel were highly remarkable for producing seven sons.

In the later ages they had fewer children, though Tolkien doesn't specify how much later or what the average was. Among human populations each woman needs to bear approximately 2.1 children in today's industrial societies just to maintain the population. In more primitive societies that number was higher due to the increased death rates. So it follows that when an Elven population achieved what might be called an "optimal level" the Elf-women would have to bear fewer children to prevent a population explosion. That might penalize the younger generations if they desired to have children, but then, they were immortal -- they could wait.

I think we can all agree that even if the Eldar of Lindon recovered some of their numbers early in the Third Age (say, in the First Millenium, before the rise of Dol Guldur), at some point their population started declining again due to the departures of Elves over Sea. Tolkien implies that an immense number of Elves were leaving Middle-earth in the years just prior to the War of the Ring. That may be the sole reason for Elrond's comment in the Council that neither he nor Cirdan any longer had the power to withstand Sauron. Or it may be that the Eldar simply never recovered their late Second Age numbers. As so many male Elves must have died in the war, their wives and perhaps many of their children might have sailed over Sea early in the Third Age.

There should be SOME reason for why the Elves didn't increase their numbers throughout the Third Age -- after all, so many Elves were departing they must have been making room for new generations. They suffered losses in the various wars, of course -- but I don't see that a decline in fertility (or fecundity) is implied. Many of the younger Elves may simply have been deferring the begetting of children because Middle-earth was becoming a more dangerous place again.

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Parma Endorion: Essays on Middle-earth, Revised Edition