If you are honest and open with your husband, then it is not a sin. If he wants more children he may take another wife.

In what circumstances are you talking about, since there are differences between a medical context and a social context.

The scholars say that the Qur'an was revealed in its entirety on Laylat al-Qadr to the heart of Prophet Muhammad (s), but the occasions on which he was to utter the revelation were done so step by step. 

If you look at all the different places in the Qur'an where it uses the root word related to 'revelation', you can see that 'tanzil' means 'revelation' (or is translated as such, but really means 'something which is sent down') or 'tanzilan', meaning 'in stages'. A 'manzil' is a stage or station (as well as meaning home/house):

This documentary from ABTV  entitled Gems From Heaven - Rings & Stones may also be useful.