Which diagnosis is most likely for a 12-week pregnant female presenting with painless spotting and a closed cervical os?

The presence of painless spotting in a 12-week pregnant female, along with a closed cervical os, is highly indicative of a threatened abortion. In the case of a threatened abortion, the key factors are the presence of bleeding (in this case, painless spotting), and the fact that the cervical os remains closed. This situation suggests that the pregnancy may still be viable, and the body has not yet started to expel the pregnancy.

The diagnosis of threatened abortion implies that while there is some risk of miscarriage, the pregnancy can often continue successfully, and many women go on to have healthy pregnancies despite early bleeding.

In contrast, an inevitable abortion would typically present with a dilated cervical os and often accompanied by more significant bleeding or cramping. An incomplete abortion refers to a situation where not all products of conception have been expelled, and this is usually indicated by the presence of a dilated cervix along with heavy bleeding. A complete abortion means that all products of conception have been expelled, which would also be signified by a closed cervix, but usually follows significant cramping and bleeding, which is not the case here. Therefore, the scenario of painless spotting with a closed cervical os aligns perfectly with the diagnosis of a threatened abortion.