Only when there is a reason to. I was diagnosed with a thyroid problem right before being pregnant and started taking medicine which meant I kept losing weight as much as I was gaining it so my weight never really changed till the second half oh the second trimester. Once I told them the reason, they understood and didn't say anything else. But then again in my case I was visibly losing weight in different parts of my body but my stomach was already getting huge with the baby so it was pretty clear I was gaining the weight I needed despite my weight.
They only mention it cause weight gain/loss is one of the things they use to track how the baby is progressing. Too much or too little could lead to complications in the end like pretern labor or baby being to big to risk natural birth for some women. Plus they want to make sure it is not something actually causing it that needs to be treated (like thyroid.) It is not always an accurate way to track it, but all the methods can be faulty, they thought my first was going to be 9 lbs based on measuring my stomach but she was only 7.6.
Nope. He don't say anything. 31 weeks, And am at my start off weight. I lost 8 lbs in my 1st trimester so am just now gaining it back. With my 1st pregnancy i gained 30 lbs an my baby was only weigh 4 pounds after me gaining all that weight.
For my daughter i lost almost 30 lbs through my whole pregnancy and they would get concerned because they said that she was little, she was born 40+1 and weighted exactly 6 lbs, but if your loosing a lot of weight they should check if the baby is gaining weight the way it should be