All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
There is nothing wrong with telling your mother the reason that made you delay pregnancy. This is not part of the secrets of marital life which are forbidden to be revealed. If the couple agrees to delay pregnancy for a specific period of time, due to some perceived benefit, there is nothing wrong with that.
However, if the woman has some fasts to fulfill from Ramadan, but does not fulfill them due to pregnancy until another Ramadan arrives, then she is excused and is not required to do anything but fulfill them at a later time.
Based on this, we do not recommend that you delay pregnancy for this reason, considering the long time you have been married. Likewise, pregnancy will be a way to dutifully please your mother and remove her worries.
For more benefit, see Fatwa 81939.
Allaah Knows best.