Like so many other couples these days, the Toronto-area business executive and her husband put off having children for years as they built successful careers. Both parents were in their 40s — and their first son just over a year old — when this spring the woman became pregnant a second time. Seven weeks in, an ultrasound revealed the Burlington, Ont., resident was carrying twins. “It came as a complete shock,” said the mother, who asked not to be named. “We’re both career people. If we were going to have three children two years apart, someone else was going to...