Window of Implantation (WOI)

The window of implantation (WOI) is the few days each cycle when the endometrium, the lining of the uterus, can accept an embryo. Progesterone prepares the endometrium. The body produces progesterone after ovulation. It comes from the corpus luteum, the small gland that forms once the egg is released. If ovulation is weak or this gland functions poorly, the endometrium changes only partially. The lining is then less prepared to accept an embryo.¹

In most pregnancies, the embryo implants 8 to 10 days after ovulation.² This timing follows ovulation, so a set cycle day can miss it. Cycle charting that identifies the peak day gives a reliable marker in natural cycles. The window then falls within the post-peak phase. After repeated implantation failure, a laboratory test of the lining (ERA) looks for a window that opens early or late. The largest randomized trial found no gain in live births from timing embryo transfer by this test.³ Some later observational studies report gains.⁴

A restorative approach asks a root-cause question. Does the body produce enough progesterone after ovulation to prepare the lining? Named methods such as NaProTechnology assess this with luteal phase evaluation and progesterone testing after the Peak Day. They use the peak-plus series. The window depends on healthy cycle function. When ovulation and the corpus luteum function well, the window can open on time.