Baby's Length

Nicole • 26 years old | Ashton 3.15.17 💙 | Kiara 3.7.18 💖
Hi ladies. I noticed that baby is still being measure from head to rump. I'm 18 weeks today and baby is measuring 5.5 inches. I was wondering if anyone knows how long baby would be if we measured from head to feet? Why isn't it measured this way even though baby's legs have lengthened now?