When Sen. Tammy Duckworth welcomed her second child, it was cause for celebration — not only because she had become the first sitting senator to give birth or because she has already begun to challenge the Senate to change its rules to allow her to bring her infant on the floor with her during voting. In giving birth to Maile Pearl Bowlsbey, Duckworth, a combat-injured veteran who has overcome considerable obstacles in her life, beat the odds yet again.

At 50, Duckworth, who graduated from McKinley High in 1985 and the University of Hawaii in 1989, became one of a growing number of women to have a child at an age once considered an unlikely, unwise or even irresponsible time to attempt new motherhood.

Times have changed.

The most recent data from the National Center for Health Statistics shows that, though the overall U.S. birthrate continues to fall, among women over 40, there has been a 4 percent increase. Can the baby boom be explained by advances in medicine? Yes and no, says Dr. Helen Kim, chief of Reproductive Endocrinology and Infertility at the University of Chicago.

“I don’t think anything medically has happened that has dramatically increased the reproductive lifespan,” she says. Instead, the change has come with another, more readily available, technology: egg and embryo freezing, and donor eggs and embryos.

“There is a huge increase in the number of people who are doing egg donation or embryo donation,” she says. “In the most recent reporting year, there were something like 20,000 embryo transfers of donor eggs or embryos. I think that’s where the bulk of these older moms are coming from.”

Duckworth has said that she used in vitro fertilization to conceive, but has not specified whether or not she used donor eggs. Other well-known women, including Janet Jackson, who gave birth last year at age 50, have adopted a similar policy when it comes to discussing the details of their treatment.

“There’s such a positive portrayal of these older women who are having babies,” says Kim, “and that’s great. But people don’t realize how rare it is and probably that they used an egg donor. I think many of these celebrities have used egg donors, and they just don’t come forward with that, and it gives people a false impression. It sort of makes you feel like maybe you can have it all, but there is still a biological clock.”

Success rates for older mothers trying IVF with their own eggs, she points out, have not increased dramatically. The real changes around older mothers are at the intersection of medical science and societal norms.

“Something that has changed in my career,” Kim says, “is the willingness of people to take care of older women. Decades ago, it was considered very controversial to even allow people who are post-menopause to access egg donation. People thought it was outrageous, ‘How can you treat someone who’s (older than) 50 with egg donation? She’s at the age of natural menopause.’ “

But fertility doctors, who have spent decades wrestling with ethical and scientific questions ranging from helping unmarried mothers to egg donation and surrogacy for gay couples to how to deal with embryos left behind after a couple splits, now view older mothers differently. In part, it’s due to a healthier population.

“People are saying, well, people are living longer now, women are healthier now,” Kim says. “There’s no evidence that someone who’s 50 is any less healthy than someone who’s 44.”

The way doctors view patient choices around life circumstances and timing has also shifted as technology to freeze eggs and embryos has improved and become increasingly prevalent.

“There are a lot of things that were considered very controversial when I was in training,” says Kim, “that now we do all the time.” The list is as wide-ranging as our lifestyle choices: unmarried couples who aren’t sure they will be together permanently but freeze embryos anyway; cancer patients; single parents. “We’re a lot more willing to use this technology that’s been around for a while for different situations. There was a time when people were like, ‘Oh that’s so outrageous,’ and now we say, ‘We can do it — why not?’ “

Similarly, the cutoff age for attempting motherhood has shifted. “When I was a resident,” Kim says, “if you had a 35-year-old mother, that was an old woman having a baby. Today that’s so normal.” Many IVF clinics once refused to treat women over 45, but today that number is more typically set at 55.

Kim says the argument that older mothers might struggle to raise children or die while they are in childhood is also a fading consideration.

“We treat cancer patients,” she says, “and there’s a big push to help those women. So, who’s more likely to die, a cancer survivor or the woman who’s 54 years old?” Bottom line? “We can’t decide for the patient; we don’t know who’s going to die.”

Past age 45, most IVF patients will undergo additional counseling and testing, Kim says, to rule out underlying health issues. Women in their 50s are more likely to have serious complications during pregnancy, and are more likely to have a cesarean section.

“They are at their end,” Kim says. “They really know their time is short, so you just really want to be careful that you don’t oversell or overestimate the likelihood of success for these people. Because I do feel like they are looking for any kind of hope, and sometimes I feel like we are preying on their emotions and vulnerability.”

Because many women now delay pregnancy to pursue a career, Kim finds that her patients may have a hard time accepting less than success.

“Usually these women have been very successful in all other areas of their life, and I think they have a hard time believing that, with all the research they’ve done, if they work hard and do everything, that they can’t make this work out,” she says.

Experimental treatments that attempt to rejuvenate the ovaries or coax stem cells into producing new eggs are much talked about on the internet but still have not yielded concrete results. So, in spite of glowing stories about new motherhood after 50 and a trend that shows it’s on the rise, Kim still offers her patients a message they’ve heard before.

“The traditional teaching is that you are born with all the eggs you’re ever going to have, you lose them over time and there’s no way to regenerate them,” she says. “That still holds. So I still counsel patients that there’s still a biologic clock, you can certainly freeze eggs or embryos for the future, but it’s not a guarantee.”