Nope, not necessary. We've never had a monitor, not even for our preemie.
When we had our first baby we were living in a small apartment and co-sleeping, so it seemed silly. There were a few times I wanted to leave baby napping inside and go out to work in the garden or hang laundry, but I found that if I opened the doors and windows between us and checked every 15-20 minutes for signs of waking, that worked just as well.
We're on our third now, and finally in a full-sized, two-story house, and still not using a monitor. Baby naps in the living room downstairs (older sibs play in the adjacent family room), which I specifically chose as the setup because the living room has a window onto the backyard, which I can leave open in case I need to go out there to deal with chickens or something.
You might talk to your mom, though, especially if she's your primary childcare provider. Maybe she would prefer to have a monitor.