For Jennifer Lawton, finding a way to have both a career and a child was simple. All she had to do was finish four years of medical school, seven years of residency in surgery and a two-year fellowship in heart and lung surgery.

That''s a total of 13 years after she graduated from college, pre-med.

Before then, Lawton, 37, said she was in no position to have a child.

"I had long hours, no sleep, and you don''t eat well," she said of her extended training as a cardiothoracic surgeon. She''s now an assistant professor of surgery at Washington University''s School of Medicine.

Lawton''s timing for having her first child supports the thesis of Abbigail J. Chiodo and Michael T. Owyang: Women who postpone having children to establish careers are more likely to earn more money over their lifetimes.

Chiodo, a senior research associate at the Federal Reserve Bank of St. Louis, and Owyang, an economist at the bank, argued their premise in the April issue of The Regional Economist.

"Time spent away from work has a negative effect on a woman''s wages, because she sacrifices valuable experience," Chiodo and Owyang wrote in the St. Louis Fed''s quarterly publication, available online at http://stlouisfed.org/publications/re/2003/b/pages/marriage.html.

A year ago in The Regional Economist, they noted that married men tend to make more money than men who have never married. They noted a strong correlation between marriage and men''s willingness to shoulder more responsibility, which is usually rewarded at work with higher pay.

This year, they pursued the female side of the equation.

What they found in reviewing several recent studies is that for women planning to "have it all," timing is everything. Many people understand this intuitively, but Chiodo and Owyang pored over studies that tried to quantify how a woman''s biology or desire to become a mother can affect her career.

Women who choose to have children in their 20s or early 30s, before they have become highly valued in their jobs, will pay a price, Chiodo and Owyang found.

"Children have direct effects toward the decreased time a woman spends on her career," Chiodo said in an interview. "Or the woman may choose not to invest in human capital, like training."

A mother with a young child or children at home, Chiodo said, is less likely to volunteer to work long hours or weekends - things that may impress the boss and make one a candidate for additional responsibilities, promotions and more pay.

"For women, marriage may signal that an employee has priorities other than work; so, an employer could interpret marriage as a signal that a woman is less reliable, less dedicated and less permanent," Chiodo and Owyang wrote.

The law prohibits employers from asking direct questions of prospective or female employees regarding their plans to have children. Yet Chiodo said that it''s hard to know exactly what goes on in a supervisor''s mind when sizing up a promotion or hiring prospect who''s a woman.

Chiodo and Owyang found that women who are married and women who are unmarried make virtually the same income in comparable careers.

And since most men don''t contribute all that much to housework and child rearing, the big variables are when a woman chooses to have a baby and how well she has launched her career before she takes time off, Chiodo and Owyang found in reviewing studies on what happens to women''s wages when they have children.

"More housework implies lower wages," said Chiodo. "More children implies lower wages."

For Lawton, planning a career in medicine - while having a family - was key from the outset of her march through the rigors of medical school and surgical training. Shooting for a high salary didn''t matter as much as finding the right time to start a family.

She and her husband work long hours and juggle their on-call and on-duty time so that one of them is at home with their son. They also have a nanny who takes care of their child when they''re at work.

"Money wasn''t a part of it," Lawton said. "I needed to finish my training."

While becoming established in the demanding field of heart and lung surgery, Lawton said, she believed she could not turn to her male colleagues to fill in for her while she took time off to have a baby or to provide such day-to-day baby care as breast feeding.

"If you take maternity leave, then the males would have to cover for you. Some of them might be enlightened, but others might not be," Lawton said.

So she put her career on hold, took a leave, now is back teaching surgery - and apparently doing just fine.

Timing was key for Renee Sevier-Monsey, 39, a senior billing administrator for Monsanto Co. She''s on her second career, after spending her 20s as a producer at KMOX radio.

Now she has a 19-month-old daughter - and a job that provides the flexibility to be a mother without bearing too much guilt for working.

Her husband, an actor with flexible hours, takes care of many of the details of running a home: scheduling the plumber, getting the carpets cleaned, taking their child to the doctor.

She has no regrets that she started a second career in her 30s and had her first child recently.

"I could make twice as much money doing something else, but it would not be as gratifying on the home front," Sevier-Monsey said. "It depends on how you balance it all."