Normal placental development and ovular anomalies during the first trimester of gestation. The normal sequences of placental development are first briefly summarized to allow a better understanding of ovular pathology. Fifteen percent of recognized pregnancies end with miscarriages, and probably one out of two conceptuses fail even before implantation. Most spontaneous abortions occur before 15 weeks of gestation, and are caused by zygotic defects: abnormal karyotypes represent more than 50% of the cases. Trisomies predominate (50 to 60%), followed by triploidy (20%), monosomy X (15%) and structural anomalies (4%). The phenotype of these lethal anomalies have been meticulously described so that the pathologist is allowed to diagnose aneuploidy on morphological features. It should be emphasized that similar abnormal phenotypes have been observed with normal karyotypes suggesting undetected genetic factors. Hydatidiform mole, twinning, whose incidence is much higher among aborted material than among live births, are other zygotic causes of abortion. Embryonic death or placental insufficiency leading to miscarriage can also result from abnormal implantation or be secondary to lesions of the membranes, as observed in amniotic band syndrome. Maternal causes of spontaneous abortions, local or general, are probably rare at this stage of gestation and difficult to diagnose by the pathologist. Retroplacental haematoma, infarcts or endometritis are often undistinguishable from leucocytic infiltrates or necrosis following the usual long period of retention.