@mazelee: As a mathematician it excits me to see how math and pregnancy work together!!! Have you every wondered why they try to guess the age of a baby in vitro? Well it turned out about 30% of mothers reported their last menstraul period (LMP) incorrectly. Most mothers thinking they are further along than they actually are. And in practice doctors usually used what the mom said over the measurements. So if a mom goes into early labor, the doctors might think the baby is more developed than they actually are. That can lead to problems if the right equipment or care for a very premature baby isn’t ready. It could also delay important treatments the baby might need right away. And not surprisingly not having accurate measures of GA affects Black babies more than others since Black babies in general have historically lower birthweights. So thats why an accurate estimate of gestational age is important to womens health! The ultrasound machine will Estimate Gestational Age and Fetal Weight by using regression equations and measurements like the babys limbs and head circumference. At this point I was 21 weeks, and my LMP calculations from my flo app and the ultrasounds calculated gestational age was only 2 days apart. But not every woman gets an ultrasound or even has access to one. Don’t forget track your periods with apps or if you don’t have that just use the old paper and pencil!