We are so happy that Jenna Bush Hager is a mom of three now, but just so she knows what she's in for with her new baby: When it comes to kids, three is the magic number...for stress.

Jenna welcomed baby Henry, a little brother for her daughters Mila and Poppy, meaning that she's joining a special, and especially tired, club.

According to a TODAY Parents survey of more than 7,000 U.S. mothers in 2013, mothers of three children stress more than moms of one or two, while mothers of four or more children actually report lower stress levels. Once you get a certain critical mass of kids, life seems to get a bit easier.

So parents like Hager, Prince William and Duchess Kate, and Kim Kardashian West and Kanye West, may want to consider having a few more kids — or at the very least learning some stress-reducing breathing techniques.

On a scale of 1-10, with 10 being the most stressed, the average mom in our survey put herself at an 8.5.

What’s stressing moms out? Plenty, from money worries to balancing the demands of work and home to feeling like husbands are sometimes just another big kid demanding attention.

But the big secret of mom stress is that a lot of it comes from within: 75 percent of mothers said they stress more about the pressure they put on themselves to be perfect than they do the pressure or judgment they get from other moms.

“You always hear about the mommy wars, but I feel like we’re judging ourselves more harshly than anyone else,” said Jill Smokler, “Scary Mommy” creator and author of “Motherhood Comes Naturally (And Other Vicious Lies).” Smokler has three kids, and totally agrees that it’s the most stressful number.

“Going from one to two was an easy, breezy transition,” said Smokler, who lives in Maryland. “Two to three, everything was turned upside down. I did not feel like I had it together...just crossing the street and not being able to physically hold all their hands I found tremendously stressful.”

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More stress nuggets from the online survey of 7,164 U.S. mothers, conducted by TODAY.com and Insight Express:

46 percent of moms say their husbands/partners cause them more stress than their kids do.

72 percent of moms stress about how stressed they are.

Biggest cause of stress: 60 percent say it’s lack of time to do everything that needs to get done.

60 percent of moms say raising girls is more stressful than raising boys.

Nine out of 10 moms stress about staying fit and attractive.

Dr. Janet Taylor, a psychiatrist in New York, says mom stress is a problem she sees daily in her practice.

“Moms are acutely aware of the fact they do not have the time to take care of their own needs,” Taylor said. "Forget reading a book, exercising or fun hobbies: Some moms barely have time to shower."

“Before you’re a mom, you take that for granted,” added Taylor. “When you are a mom you just don’t have the time.”

She laughed when she heard that having four or more kids was less stressful than three: She’s a mother of four, including a set of twins — and agrees with the survey findings.

“There’s just not enough space in your head for perfectionism when you get to four or more kids," she said.

For example, Taylor recalls with her fourth child she didn’t bother with things like obsessively covering all the outlets with safety plugs. “It just gets to be survival!” she joked.

Plus, she thinks moms hit a groove once they get past the outnumbered phase of having three kids and into the seriously outnumbered territory of four or more.

“The more children you have, the more confident you become in your parenting abilities,” she said. “You have to let go… and then you’re just thankful when they all get to school on time.”

Taylor’s children are now adults and says while the stress differs from the hands-on parenting years, it always remains. “Now I get stressed out by things like my oldest having a job interview and my youngest being in the middle of finals,” she said. “I’m on the other side… hopefully you can feel like you’ve prepared them well.”

Taylor says daily stress levels of 8.5 on a scale of 10 — the average that moms in our survey report — take a toll on mind and body. She recommends her stressed-out patients try this exercise: Take five minutes and draw a pie chart showing how you actually spend the hours in your day. Then flip the paper over and draw a pie chart of what you’d like to be doing. Pick one of the things that’s on chart two (what you want), but not chart one (the reality), and figure out a way to make it happen.

“You have to be able to say no to your kids, to your spouse, to your friends sometimes,” Taylor said, explaining that many women find that part really hard. “Instead of making the perfect lunch for your kids, go for a walk by yourself. Even if it’s just 10 minutes, take some deep breaths and focus on what you need.”

Connecticut mom Karen Hobert Flynn says saying "no" to some kids’ activities was one sanity-saver as a mom of many. Her four boys now range in age from teens to adults, but her rule when her kids were younger was one after-school activity per child.

“You can do Boy Scouts or a sport. We didn’t do five sports. We said ‘no’ to intense travel teams,” she explained. And she said it didn’t limit her boys; they all played competitive team sports at school as teens.

Having four had its advantages, she said. Each child had a built-in playmate; they tended to pair up so no one was left out. And her backyard was always full of kids, even though they had far from the fanciest swingset in the neighborhood.

“We had critical mass. Kids in the neighborhood would want to come here because it was an immediate party,” she said.

For her and her husband, going from one child to two was a big adjustment; transitioning from two to three, “you’re outnumbered,” and adding a fourth child was “not as big a jump.”

She says when her boys were young, she kept stress at bay by staying organized, connecting with other moms, cultivating good babysitters, relying on her husband — and of course, not taking life too seriously.

“We laugh a lot,” Flynn said. “It’s a lot of fun.”

This story was originally published on May 6, 2013.