['To study serum levels of placental protein 14 (PP14) in relation to endometrial function in women with a history of habitual abortion.', 'Prospective study.', 'Departments I and II of Obstetrics and Gynecology, University Central Hospital of Helsinki, Helsinki, Finland.', 'Fifty patients (26 primary and 24 secondary habitual aborters) and 38 controls without a history of abortion studied during a regular cycle.', 'Habitual aborters as a whole or when subgrouped into those with normal cycles (n = 40) or with a luteal phase defect (LPD; n = 10) and control women demonstrated a distinct increase in PP14 levels from late follicular to late luteal phases. In the luteal phase, serum PP14 levels were lower in the patients than in the controls (27.2 +/- 3.1 versus 48.5 +/- 10.1 micrograms/L), but the differences in PP14 levels between habitual aborters with or without LPD was not significant (16.3 +/- 4.3 versus 29.9 +/- 3.7 micrograms/L).'] Is habitual abortion accompanied by low serum levels of placental protein 14 in the luteal phase of the fertile cycle?