Bonnin M, Canivenc R, Ribes C
J Reprod Fertil. 1978 Jan;52(1):55-8. doi: 10.1530/jrf.0.0520055.
There was a biphasic pattern of progesterone secretion during the year. Delayed implantation was characterized by low concentrations from February to June, a significant (P less than 0.001) increase during July, August and September, and a return to low levels in October-November. A second significant increase (P less than 0.001) was observed in December and early January just before the presumed time of implantation.
