My first pregnancy I had implantation bleeding so my "period" was unreliable for calculating due date - and I'm worried this pregnancy (2nd) is too. Just had my first ultrasound and they measured at a week earlier than they had calculated based on my last period. But my ultrasound looks similar to ultrasounds that are farther along (about three weeks farther along at 16 weeks?) Instead of ultrasounds done at 13 weeks. I'm wondering how likely the possibility is that I had implantation bleeding again and I'm actually farther along and the baby is just measuring smaller? Probably just being paranoid though lol.