An improvement in clinical pregnancy rate follows a natural or induced period of amenorrhoea. The cause is unknown, but it could be related to the period of uterine 'rest' which may restore full function to the steroid-sensitive systems, such as the pinopodes, after a prolonged period of constant menstrual cycling. This may result in the restoration of uterine capacity for embryo implantation to levels typical of younger women. Some other avenues of research on the unusual aspects of human implantation are worth further study. An impaired uterine blood flow could lower the chances of implantation. Immunological factors may emerge as major elements in the human implantation process, because the neutralization of antiphospholipid antibodies may enhance the activity of early trophoblast.