Purpose: To evaluate the minimal endometrial thickness at the time of ovulation at which conception is possible.
Methods: Treatment with luteal phase progesterone support after oocyte release despite attainment of a maximal thickness of 4 mm.
Results: A pregnancy and successful delivery were achieved.
Conclusions: Pregnancy is possible with only a 4 mm peak endometrial thickness, at least when follicle maturing drugs are not used.