During the first trimester of pregnancy extravillous trophoblast cells (EVT) invade the maternal decidua. Invasion normally is reduced from the second trimester onwards and stops in the inner third of the myometrium. By contrast, in extrauterine tubal pregnancy, trophoblast invasion may even penetrate the tubal wall, which ultimately leads to the rupture of the fallopian tube. Induction of apoptosis of EVT cells, by maternal immune competent cells, may be an important mechanism to limit EVT invasion in uterine pregnancy. Tissue specimens from first and second trimester uterine pregnancy and first trimester tubal pregnancy were analyzed for apoptosis by TUNEL- and M30-staining. By immunohistochemical double labelling, maternal leukocyte subtypes were co-localized to apoptotic cells and in this context, the number of CD56(+)NK cells was analyzed. Our data show that apoptosis is confined to the decidua basalis. Most apoptotic cells are single cytokeratin-positive epithelial cells residing in the stromal compartment. Consequently these cells can only be EVT cells. Maternal leukocytes are not apoptotic. They are located in close contact to apoptotic cells. The number of apoptotic cells in the second trimester (1.8+/-0.7 per cent) is reduced compared to first trimester (5.6+/-0.7 per cent) of uterine pregnancy. In parallel, the number of NK cells declines from first (24.4+/-2.9) to second (12.4+/-1.8) trimester. Furthermore, apoptosis is significantly reduced in ectopic (0.9+/-0.3 per cent) compared to eutopic first trimester pregnancies. Consequently, we suggest that in first trimester uterine pregnancy, induction of EVT cell apoptosis by the maternal immune system is one mechanism to limit EVT invasion. During the second trimester, in parallel to declining numbers of NK cells, the mechanism changes. However, in tubal pregnancy due to differing immunological microenvironments at the ectopic implantation site, apoptosis induction fails, which deleteriously may result in uncontrolled invasion and penetration of the tubal wall.