Which of the following conditions is the most frequent cause of spontaneous aboion in the first trimester of pregnancy?

At least 10% to 15% of normally feilized and implanted ova are lost in the first trimester of pregnancy because of spontaneous aboion. Studies using immunoassay of human chorionic gonadotropin (hCG) for early diagnosis of pregnancy suggest that the percentage of feilized ova lost in the first trimester might be even higher. The great majority of these cases are attributable to chromosomal abnormalities. Chromosomal studies are not routinely performed in such cases, but they are recommended when a malformed fetus has been identified or when habitual or recurrent aboions occur. Abruptio placentae, a complication of the third trimester, occurs when the placenta detaches prematurely from its implantation site. Retroplacental hemorrhage develops within the space between placenta and uterine wall, leading to interruption or severe reduction in the blood supply to the fetus. Chorioamnionitis, a complication of the second and third trimesters, results from ascending infections through the vaginal canal. Infection of chorioamniotic membranes may lead to premature rupture of membranes and aboion or premature labor. Placenta pre is a placenta implanted in the lower segment of the uterus. When dilatation of this segment begins in late pregnancy, a placenta pre may cause severe bleeding and lead to premature labor. Ref: Kipps T.J. (2010). Chapter 94. Chronic Lymphocytic Leukemia and Related Diseases. In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds), Williams Hematology, 8e.