Gestational age

So quick question and slight concern. According to my last period, I am 12 weeks pregnant. I had my first ultrasound today and baby is measuring 10 weeks 5 days. They said it could be due to either ovulating late or implanting late. I’m just concerned because I don’t feel like that answer matches up. I’ve know I was pregnant from 4 weeks, 5 days (according to my last cycle), but that would mean I was only 3 weeks when I first tested positive if I was 10 weeks 5 days. I’m just confused and concerned that my baby might just be very small. I’m concerned I’m doing something wrong or about birth defects. Help!

*side note: I was over the moon to get to see our nugget and hear it’s perfect heartbeat! Heartbeat is 184.