Morgoth's Ring, 1st paragraph, page 212.
As for the begetting and bearing of children: a year passes between the begetting and the birth of an elf-child, so that the days of both are the same or nearly so, and it is the day of begetting that is remembered year by year. For the most part these days come in spring. It might br thought that, since the Eldar do not (as Men deem) grow old in body, they may bring forth children at any time in the ages of their lives. But this is not so. For the Eldar do indeed grow older, even if slowly: the limit of their lives is the life of Arda, which though long beyond the reckoning of Men is not endless, and ages also. Moreover their body and spirit are not separated but coherent. As the weight of the years, with all their changes of desire and thought, gathers upon the spirit of the Eldar, so do the impulses and moods of their bodies change. This the Eldar mean when they speak of their spirits consuming them; and they say that ere Arda ends all the Eldalie on earth will have become as spirits invisible to mortal eyes, unless they will to be seen by some among Men into whose minds they may enter directly.

Morgoth's Ring, 2nd paragraph, page 212
Also the Eldar say that in the begetting, and still more in the bearing of children, greater share and strength of their being, in mind and in body, goes forth than in the making of mortal children. For these reasons it came to pass that the Eldar brought forth few new children; and also that their time of generation was in their youth of earlier life, unless strange and hard fates befell them. But at whatever age they married, their children were born within a short space of years after their wedding. (Short as the Eldar reckoned time. In mortal count there was often a long interval between the wedding and the first child-birth, and even longer between child and child.) For with regard to generation the power and the will are not among the Eldar distinguishable. Doubtless they would retain for many ages the power of generation, if the will and desire were not satisfied; but with the exercise of the power the desire soon ceases, and the mind turns to other things. The union of love is indeed to them great delight and joy, and the 'days of the children', as they call them, remain in their memory as the most merry in life; but they have many other powers of body and of mind which their nature urges them to fulfil.

Morgoth's Ring, 1st paragraph, page 213
Thus, although the wedded remain so for ever, they do not necessarily dwell or house together at all times; for without considering the chances and separations of evil days, wife and husband, albeit united, remain persons invididual having each gifts of mind and body that differ. Yet it would seem to any of the Eldar a grievous thing if a wedded pair were sundered during the bearing of a child, or while the first years of its childhood lasted. For which reason the Eldar would beget children only in days of happiness and peace if they could.