1. 1 Trends in fertility and infant and maternal health - United States, 1980-88 . MMWR Morbidity and Mortality Weekly Report1991 ; 40 :381 -383 , 389-390.
2. 2 E Amon . Premature labor . In:Medicine of the Fetus and Mother.Editors: Reece EA, Hobbins JC Mahoney MJ, Petrie RH. Philadelphia, PA: J. B. Lippincott, 1992 ; pp. 1398 -1429.
3. The repetition of spontaneous preterm labour