Gynaecology normally means treating women who aren't pregnant, while obstetrics deals with pregnant women and their unborn children, but there is lots of crossover between the two. For example, women may be referred to gynaecologists in the earlier stages of pregnancy, and obstetricians later in their term.
Specialises in the care of women before, during and after their pregnancies. An obstetrician is also a surgeon which means that they are able to perform a Caesarean section or other similar surgery if necessary. They can perform tests during the ante-natal period and are also on hand during delivery to monitor the process as well as dealing with any unexpected issues.