Measuring behind

Okay so my baby girl was measuring 2 days behind on my first ultrasound by her head but her weight and everything was fine. Same thing second ultrasound. She was five days off by her head but weight and everything was on time. This time she’s measuring 12 days off by head but her weight and everything is on time? I feel like she’s just tiny because me and my husband both are. I have plenty of fluids and everything. Nothing is wrong besides her head measuring small. Is my baby just tiny? I’m waiting to see what my doctor said about my ultrasounds tomorrow but I can’t help but stress about if it’s microcephaly or something else.