In a pregnant woman with painless spotting at 12 weeks gestation and a closed cervical os, what is the likely diagnosis?

In a scenario where a pregnant woman presents with painless spotting at 12 weeks of gestation and has a closed cervical os, the likely diagnosis is threatened abortion. This condition is characterized by vaginal bleeding during the first half of pregnancy, where the cervix remains closed, indicating that the pregnancy may still continue.

Painless spotting can occur for various reasons, but the absence of cervical dilation or effacement suggests that the body has not yet initiated the process of miscarriage. In cases of threatened abortion, it is still possible for the pregnancy to continue successfully, often with careful monitoring and management.

The diagnosis is distinctly different from inevitable, incomplete, or complete abortion. Inevitable abortion typically presents with cervical dilation and possible rupture of membranes, signaling that a miscarriage is underway. Incomplete abortion involves some products of conception being expelled while others remain, commonly occurring with an open cervical os. Complete abortion indicates that all products of conception have been expelled and the cervix would typically be closed after this process.

Thus, the key features of painless spotting and a closed cervical os strongly indicate threatened abortion as the correct diagnosis in this case.