Question About Ultrasounds

Lucy 👧🏻👧🏻👼🏻🤰🏻👩‍❤️‍👨

I am wondering how ultrasound techs measure baby’s weight. I understand that it’s estimated, but I just wonder what it’s based off of. My baby was measuring 6 lbs 7 oz at my last ultrasound (35+6) and that was almost an entire pound more than the previous week!