- Ob, Ob River - a major river of western Siberia; flows generally northward and westward to the Gulf of Ob and the Kara Sea
--1 is a kind of river
--1 is a part of Siberia - obstetrics, OB, tocology, midwifery - the branch of medicine dealing with childbirth and care of the mother
--2 is a kind of medicine, medical specialty
--2 has particulars: fetology, foetology; perinatology