Many women who cannot get pregnant or who may experience repeated miscarriage often suffer from a luteal phase defect, which simply means that the luteal phase of their cycle (the time between ovulation and their menstrual flow) is too short to develop a thick enough uterine lining to support a fertilized egg.
This can either result in an inability for the fertilized egg to implant in the first place, or an inability to thrive, thus resulting in a very early miscarriage. Some women actually miscarry every month without realizing it believing that they are simply having a regular period.