They were just ideas, I would imagine fetus size close to conception date would be the most accurate time you could measure a fetus. The longer the fetus was inside the more factors would lead to different sizes like u say the birth rates vary significantly, but weight and length near conception have to be very similar across the board. if the fetus was 6 inches long odds are u could tell dang near when it was conceived. I have killed does in 5 counties that had twin fetuses the last week of january, those counties are cullman, walker, perry, tallapoosa, and shelby.