Pregnancy

Lynfield [143] found more scalp follicles were in anagen during the second and third trimesters (95%) and for about a week after birth; by six weeks this fell to about 76%, remaining low for 3 months. Pregnancy hormones maintain follicles in anagen, but after birth many enter catagen and telogen, causing a synchronised partial shedding or moult. This may be particularly noticeable in autumn due to seasonal shedding (Section 1.6.2). Which hormones are involved is uncertain, although oestrogen and prolactin are possibili­ties. Human follicles have prolactin [144] and 17^-oestradiol [126,127] receptors, but 17^- oestradiol inhibits cultured human follicles [145], and rodent hair growth, accelerating catagen onset [121-123], the opposite of the pregnancy effect. Prolactin reduces human follicular growth in vitro [144] supporting a role in post-partum shedding.