The doctor chose the heir?

The announcement that Lady Cholmondeley was expecting twins was revealed by Richard Kay of the Daily Mail^[ and Mandrake of The Daily Telegraph. On 12 October 2009, the Marchioness gave birth to twin sons, Alexander Hugh George and Oliver Timothy George, who were originally expected in January. The birth took place shortly after Lady Cholmondeley was rushed to Queen Charlotte's Hospital in London, where she was monitored for two weeks. The elder son, Alexander, as heir apparent to the Marquessate, bears the courtesy title of Earl of Rocksavage; his brother is known as Lord Oliver Cholmondeley.

Assuming that the delivery was by caesarian section, then the difference between being known as The Marquess of Cholmondeley and merely Lord Oliver was a matter of minutes and purely at the discretion of the delivering doctor.