The science behind Janet Jackson’s pregnancy at 49

Her tour might be called Unstoppable, but Janet Jackson has decided to put it on hold. That’s because the pop star is pregnant just weeks before her 50th birthday, according to the celebrity news site Entertainment Tonight.

That will make hers a rare quinquagenarian pregnancy. But she’s one of a growing number of older new mothers in the United States — a trend that’s bolstered by changing societal norms and new advances in fertility treatments.

Here are three things to know about pregnancy post-50.

1. Pregnancies after 50 are rare, but they do happen

The number of births to women of this age has jumped significantly: 743 women aged 50-54 gave birth in 2014, up from 255 in that age group a dozen years earlier, according to data from the Centers for Disease Control and Prevention. But it’s still exceptionally rare: only 1 in 10,000 births in the US are to women Jackson’s age.

There’s an overall trend toward older motherhood in the US. A growing share of these are first-time births, reflecting the fact that women, like Jackson, are waiting longer to start families.

Among celebs, Jackson is probably the most noteworthy post-50 mom, though many stars — including Geena Davis, Sophie B. Hawkins, and Halle Berry — have had children in their late 40s. But Indian villager Rajo Devi wins the prize: She was 70 when she had her first child. As Jackson herself has put it, “Dammmn, baby.”

2. It’s probably a donated egg

“It’s exceptionally rare for patients to get pregnant naturally at 50 or over 45. They make history,” said Dr. David Keefe, an obstetrician-gynecologist and fertility researcher at New York University.

In part that’s because around age 50, many women are entering menopause, after which egg harvesting isn’t possible.

And as eggs age they also accumulate more genetic glitches. The mechanism is not completely understood, but it may be because eggs have shorter telomeres, the protective caps on the ends of our chromosomes.

Once the egg’s DNA has accumulated damage it can’t repair, then its division as an embryo goes haywire, with chromosomes sticking together when they should come apart. “The resulting embryo may end up with too many or too few chromosomes,” said Dr. Kutluk Oktay, a fertility doctor and researcher at the New York Medical College. “The majority of those end up in miscarriages.”

So older would-be mothers often use eggs donated by younger, more fertile women. For younger women seeking pregnancy, donor eggs might only be discussed after they’ve tried other kinds of fertility treatments, but for those over 45, “that’s kind of where they start,” said Alicia Abdella, an infertility social worker at Massachusetts General Hospital.

3. It’s the age of the egg rather than the age of the womb

According to the Society for Assisted Reproductive Technology, “the single most important factor in predicting the success of [in vitro fertilization embryo transfer] is the age of the female partner.”

The organization has said that the likelihood of pregnancy using young donor eggs in an older mother are comparable to those of young women using their own eggs.

Still, advancing age may make gestational surrogates an attractive alternative, since older women do face increased health risks with pregnancy, including gestational diabetes, high blood pressure, and premature birth. Cost can be one of the biggest barriers to gestational surrogacy, but for people for whom money is no object, it is an option.