Sadiq, M. E., A. L. Abubakar, A. Lawal, A. H. Ahmad, and N. Sada. “Preterm Deliveries and Progesterone Levels in Pregnant Women Attending Antenatal Care in a Health Care Facility in Sokoto State, Nigeria: A Preliminary Study”. International Journal of Reproduction, Contraception, Obstetrics and Gynecology, vol. 9, no. 7, June 2020, pp. 2969-72, doi:10.18203/2320-1770.ijrcog20202742.