There are a number of reasons for the shift in ages of new mothers. Experts do caution that there are increased risks with later pregnancies.

Julia Van Buren is the mother of three young boys.

She and her husband are raising their family just outside of Boise, Idaho.

The couple dated for five years before they got married. Van Buren said they decided to tie the knot when they decided they wanted to have a baby.

And she didn’t experience that feeling until she turned 30.

“I just didn’t think about it. I was having fun, traveling, working,” she said. “It didn’t become a real priority to me, until one day I was ready.”

For many women like Van Buren, the decision to postpone childbirth until their 30s is becoming more common in the United States.

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A report this month from the Centers for Disease Control and Prevention (CDC) confirmed that for first time women in their 30s are giving birth at a higher rate than women who are in their 20s.

The analysis showed the birth rate among women ages 30-34 increased from 101.5 per 1,000 in 2015 to 102.6 births per 1,000 in 2016.

The birth rate for women ages 25-29 fell from 104.3 per 1,000 in 2015 to 101.9 per 1,000 in 2016. This age bracket had held the top spot for three decades.

Additionally, the birth rates for women in the age brackets 35-39 and 40-44 also increased slightly, while the birth rate dipped for women 20-24.

The CDC also revealed that the average age of first-time moms is now 28. About 50 years ago, the average age was 21.

The changes in demographics can be tied to number of reasons, according to the medical experts that Healthline interviewed for this story.

“I think there are multiple factors playing a role here. We are certainly seeing a shift in societal norms, with more women deciding to complete their education and start their career before they think about getting married and/or starting their family,” said Dr. Molly Moravek, an assistant professor of obstetrics and gynecology at the University of Michigan, in an email to Healthline. “I think improved access to safe and effective contraception, particularly long-acting reversible contraception, has contributed to this trend as well. These factors have let women think about when the ‘best’ time is for them to have children instead of when they ‘should.’”

Despite the shift in demographics, science still says that women in their 20s are at the optimal age to give birth.

They tend to have the lowest risk and fewest complications during the gestational period.

But as Moravek mentioned, societal norms and expectations surrounding women and childbirth are evolving.

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Van Buren, who is originally from California, said most of her friends from college waited to have kids in their 30s.

But where she lives today, that is not the norm.

She attributes the swell of younger mothers in her community due in large part to the population of Latter Day Saints living near Boise. Many couples who practice the Mormon religion tend to marry in their early 20s and start families right away.

Most of her boys’ classmates have mothers who are much younger than she is.

“I’m an old mom,” Van Buren said with a chuckle.

While all of her pregnancies — at ages 31, 33, and 38 — were considered healthy, Van Buren did have her last baby during what doctors call “advanced maternal age”, meaning she had a baby while over the age of 35.

With age comes greater risks for health issues such as hypertension and gestational diabetes, according to Dr. Katherine Economy of Brigham and Women’s Hospital in Boston.

The risk of stillbirth is also a serious issue, she added.

Another big concern for babies born to women over the age of 35 is the risk of Down syndrome.

According to the March of Dimes, the chance of having a baby with Down syndrome increases exponentially once a mother reaches her mid-30s. At age 25, the risk is about 1 in 1,350. At age 35, that risk jumps to about 1 in 350.

Economy noted that while the risks associated with an older mother are real, it’s important to keep them in perspective.

“All of the risks increase, but they are gradual increased risks,” she told Healthline.

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That doesn’t mean that these issues will occur in every woman who gives birth after the age of 35. Nor does it mean bad news if a mother does develop some of these risks.

“It’s about counseling,” Economy said. “Most women [who develop these risks] will be managed in a high-risk setting.”

Despite the fact that more older women are giving birth, the risks associated with that won’t change, according to Moravek.

“Unfortunately, biology has not caught up with society,” she said. “Younger women still have higher pregnancy rates and lower miscarriage rates than older women.”

Both doctors agree, though, that the report makes a good case for social policy to catch up with the current times.

Economy said she’d like to see a more robust maternal health plan for women and see the country move toward a national paid family leave policy.

Right now, the United States is one of just a handful of countries around the world that doesn’t have a federal work-leave law in place.

“We need a more supportive approach,” Economy said. “And better social programs.”