The Sequence of Lanugo Pattern Development on the Trunk Wall in Human Fetuses

The study examined the development of lanugo, a type of fetal hair, in 278 human fetuses ranging from 69 to 226 days of gestation. It was found that the first single hairs appeared on the posterior trunk wall in the 17th week of fetal life and on the anterior wall between the 18th and 19th weeks. Lanugo appeared significantly later on the anterior trunk than on the posterior, with a nearly 2-week difference. The full hair pattern was observed on the posterior wall by the 19th week and on the anterior wall by the 21st week. The lanugo development process was completed on the posterior wall by the 23rd week and on the abdomen by the 26th week. The study concluded that the lanugo developmental cycle, involving the appearance of initial hairs, partial hair, and final patterns, was consistent on both trunk walls.