Final answer:
The 'estimated fetal length' is the rule of thumb used for estimating fetal development from crown-to-heel length measurements.
Step-by-step explanation:
The rule of thumb for estimating fetal development from crown-to-heel length measurements is known as the estimated fetal length. During the fetal growth period, measurements such as crown-to-rump length in the early stages and crown-to-heel length later in development are used as predictors of gestational age.
The development of the fetus is meticulously monitored through these measurements, with the average crown-to-rump length immediately before birth being 35.5-40.5 cm (14–16 in) and the fetus weighing approximately 2.5-4 kg (5.5–8.8 lbs). After birth, the baby's length is measured from head to toe since they are no longer confined to the fetal position, with the average newborn length being approximately 51 cm (20 in).