Oh my god!! Your MIL needs to buzz off what a wicked woman!!
First of all congratulations on your pregnancy and looking after that little miracle so well.
Having fairly recently had a baby (she's 6 months old now) I can honestly say I never, not once, weighed myself while pregnant. I have no idea how much weight I put on but am very active and went to the gym every day throughout. I knew I'd obsess with it and at the end of the day most of the weight is unavoidable. I ballooned towards to the end, had a chubby little face and chunky thighs, but it was all WATER!!
I was back in my normal clothes after a week or so. Didn't even wear jeans for months after anyway as lived in comfies (leggings, big knickers - the aftermath is uncomfortable I'm afraid!)
Much if it is taken up by the baby, water, blood (this increases by around 45%) I'm sure very little will actually be "fat". And once baby is born you'll lose the water weight.
Don't let one ignorant comment make you anxious. Even if you have put on a little weight, who cares?!!! Your body is creating this wonderful little being who you're soon to meet, once they're here you won't think about it and you'll be running around in no time taking baby out in the pram etc it'll just drop off you.
But I'm begging you not to let this get into your head.
That last thing your newborn needs is you to worry about your weight and watch what you eat. Your baby needs you to be healthy and you need the energy for the night feeds 😉