Babies change the equations. Not just babies. Babies grow into school children. School children become teenagers. This doesn't mean a mother can't work, but it does mean that the more demanding the job, the less she has left over for her children.
Children chill when the hearth grows cold.
A woman can aim to be general, a CEO of World Class Widgets, Inc., even the president of the United States, but the climb requires castoffs and often the castoffs are the ones she needs most and who need her most. Many female officers complain they're held to a higher standard than men, and are kept off the fast track. Only 68 of the 3,628 colonels are women. But that wasn't Colonel Beard's complaint. She chose a higher standard for herself and her family.