One woman was on her second career, married to a man who already had children and certain that she didn't want a child of her own. Then she hit 45, and suddenly having a baby was the only thing that mattered.
Other women were busy with medical, law or graduate school and then long hours at the office. Their 40s just seemed to sneak up on them.
Some of them spent years looking for a partner before choosing to become a single mother, or they needed time to get used to the idea of a lesbian couple having a child.
Dr. Ingrid A. Rodi, a Santa Monica, Calif., fertility specialist, understands why many of her patients have delayed childbearing until their 40s. At 53, she has lived through the social and medical revolutions fueling today's over-40 baby boom.
"Egg donation and fertility treatments make having children in your 40s more possible," she said. "But you start getting into progressive difficulties for mother and child."
The advice Rodi dispenses hasn't changed much from the advice given to her own mother more than 50 years ago: If you're in your early 30s and want to have children, it's best to get the show on the road.
Older mothers are more likely to develop high blood pressure and gestational diabetes and to give birth to premature and low-birth-weight babies. Even the use of donor eggs does not guarantee a healthy child - or pregnancy.
Women begin to have fertility problems about 10 to 15 years before they experience menopause, Rodi said. The average age of menopause is 50 to 52, but it can range from 40 to 60. Women have no way of knowing for sure where in the spectrum they'll fall.
A string of well-publicized celebrity post-40 pregnancies makes it look easy: In February, Desperate Housewives star Marcia Cross gave birth to twins at age 45. Actress Holly Hunter became a mother of twins last year at 47. Geena Davis had her twins at 48 in 2004.
But even in vitro fertilization rates decline with age when older women use their own eggs. (The newest fertility breakthrough, announced earlier this year, might allow younger women to freeze eggs for use later.) According to the national Centers for Disease Control and Prevention, the average chance for a 40-year-old woman undergoing assisted reproductive therapy to become pregnant using her own eggs was 23 percent and to successfully carry the pregnancy through to birth, about 16 percent. Both rates dropped steadily with each one-year increase in age.
Using donor eggs, however, raises the chances for a successful live birth to 51 percent for women over 40, according to the CDC.
"Because it's so reliable, the number of women getting egg donation is going up every year," said Rodi. "And that is contributing significantly to the number of pregnancies in women in their 40s."
Donor eggs also eliminate some of the risks associated with older childbearing, said Dr. Richard Paulson, chief of the Division of Reproductive Endocrinology and Infertility at the University of Southern California's Keck School of Medicine. Aging eggs not only contribute to infertility and miscarriages but are more likely to have chromosomal abnormalities.
Despite the advantages, admitting to using donor eggs is one of the few remaining reproductive taboos - among celebrity and regular mothers alike.
Donor eggs aren't foolproof - nor are they cheap. Treatment can cost from $10,000 to more than $25,000 per attempt, with high-demand donors such as Ivy League graduates and models commanding higher fees.
What's more, "virtually every complication associated with obstetrics is increased with increased maternal age, if you look at it statistically," said Paulson.
"But most of us believe that if women get good prenatal care and are carefully followed, the outcome is very good."
Dr. Alan R. Fleischman, medical director for the March of Dimes, an organization primarily concerned with the eradication of birth defects, is less optimistic.
"All of the complications of pregnancy - high blood pressure, preeclampsia [a rapidly progressing hypertension that affects mother and fetus], fetal death, prematurity, low birth weight - occur at higher rates in older women than in younger women," he said. "That's true with or without assisted reproductive technologies."
A 2004 study of Swedish women found the rate of premature births for women ages 40 to 44 to be 150 percent higher than for women 20 to 29 years old, Fleischman said.
Prematurity is the leading cause of infant mortality in the United States, accounting for more than a third of all infant deaths, according to the CDC. Even those born just a few weeks early - from 34 to 36 weeks - are six times more likely than full-term babies to die during their first week of life, according to a study by the March of Dimes published in the November Journal of Pediatrics.
These babies are also at greater risk of having breathing problems, feeding difficulties and trouble regulating their body temperatures, Fleischman said. Very premature babies are more likely to have developmental problems, from minor to severe retardation, and to be disabled.
The March of Dimes strongly recommends pre-conception counseling for older women, and if the woman decides to move forward with pregnancy, to have her obstetrician assess her risk of diabetes and high blood pressure. Before trying to conceive, the agency recommends that a woman get into the best physical shape she can and take daily vitamins, including folic acid, which has been shown to prevent birth defects. If she is planning on using reproductive technology, she needs to understand the risks of multiple births.
Some fertility experts believe awareness of the risks is already rising - and that the boom in over-40s births might have peaked.
"What I'm curious about is whether this trend is going to continue or whether it is, in fact, going to reverse," Paulson said. "We've raised the consciousness of the population to the fact that the biological clock is a reality."
Mary Engel writes for the Los Angeles Times.