Pizza BMI is bollocks all in all, I obviously don't know what you look like or how you are built, the MW does. If someone had a BMI of 35 for example, but obviously had a muscular physique (so not obese, just heavy) and the MW added a cm to their height.... I would think 'good on them' I may be out of order here though so others feel free to correct me.
I DO think you need to make sure you are eating a good diet though, and getting 2,000 to 2,200 healthy calories a day, as in fresh food etc, rather than donuts, chocolate and pizza etc if you can, but to be honest as long as you are eating enough and gain the weight expected in the next few weeks and baby is growing ok, I don't think it will matter.
Just keep an eye on it. Baby will take from you all that they need, even if it is at your expense so they will be fine, but you will need to build up some reserves. It is a marathon not a sprint.
If you are anything like me though, in a few weeks you are going to be HUNGRY! all the time for all of the food. MW probably knows this too.