Why did the midwife& doctor give me more folic acid?

The midwife at my first booking said I need extra folic acid I'm (nearly 9) weeks pregnant.. she said a lot of other things and I completely forgot to ask why.....

The doctor called me the next day and said he's going to prescribe me extra folic acid but I can take it with my regular pregnancy vitamins. Again, I didn't ask why because he was very busy and didn't give me the opportunity to ask any questions.. rude!

Could it be because I'm overweight and have a high bmi? I'm confused