Placental weight loss in late pregnancy

Summary. In two strains ofmice, mean placental weight was less on the 19th than on the 18th day of gestation. Although the foetus is gaining weight rapidly towards the end of gestation, in most species examined the placenta reaches its maximum weight well before parturition (see Barcroft, 1946). In the mouse, after allowance had been made for number of implants, foetal and placental weights on the 18th day ofgestation were positively correlated when the two uterine horns of individual females were compared (that is, the side with the larger placentae tended also to have the larger foetuses), but negatively correlated when comparison was made between different females (McLaren, 1965). This suggests that the mouse placenta actually loses weight towards the end of gestation: if physiologically more advanced pregnancies tended to have larger foetuses and smaller placentae the negative correlation would be accounted for. To see whether the postulated weight loss could be detected directly, females of the C57BL/McL inbred strain were paired with males of their own strain an