On average, Miller has found in a new paper, a woman in her 20s will increase her lifetime earnings by 10 percent if she delays the birth of her first child by a year. Part of that is because she'll earn higher wages—about 3 percent higher—for the rest of her life; the rest is because she'll work longer hours. For college-educated women, the effects are even bigger. For professional women, the effects are bigger yet—for these women, the wage hike is not 3 percent, but 4.7 percent.
So, if you have your first child at 24 instead of 25, you're giving up 10 percent of your lifetime earnings. The wage hit comes in two pieces. There's an immediate drop, followed by a slower rate of growth—right up to the day you retire. So, a 34-year-old woman with a 10-year-old child will (again on average) get smaller percentage raises on a smaller base salary than an otherwise identical woman with a 9-year-old. Each year of delayed childbirth compounds these benefits, at least for women in their 20s."
No wonder women ( and their husbands) agonise over when to start their family. Career versus Baby ?