I’m pro-life, but I see March 25 as a Catholic tradition. Look at all the other months that have been suggested for the birth of Jesus... how could you choose the “real” day of the Annunciation? My family celebrates Christmas on Dec 25, but my children understand that it is not a known fact that Jesus was born on this date, we don’t have any “happy birthday Jesus” cakes. That is just the day we, like the majority of Christians, chose to celebrate that event. But from there, to set a date for conception... maybe that’s fine with you as a Catholic.
If you can understand that Dec 25 is not likely Jesus’ birthday, but is the day we celebrate His birth, then you can understand that a day to celebrate His Incarnation in the womb is not the same as saying “this is infallibly the day Mary became pregnant.”
I am not suggesting a new holiday. It’s already on the church calendar.