The toughest time as a mother is when your child is a newborn, right? The sleepless nights, the crying, the vomit and poo, the shock of it all.

Yeah, it's tough. It's stressful. It's exhausting. But apparently, there's a tougher time.

A recent study in the U.S. has found that mothers of 11- and 12-year-olds suffer the lowest levels of and the highest levels of stress.

At first I found this hard to believe, but the more I thought about it, the more sense it started to make. When my kids were babies, I could almost always make things right for them. Most of their problems could be solved by feeding or cuddling or a clean diaper. It was hard, but it was doable.

Now, my daughter is nearly 10, and she still keeps me awake at night, but for different reasons. She wants to do more things on her own and have her own space.  I worry that she's going to stop being so chatty with me, and won't want to hug me and snuggle me, like she does now. I worry that she's going to have a hard time at school over the next few years, as her classmates get older and more judgmental. I worry about puberty and the complications that's going to bring. I worry about all the problems I'm not going to be able to solve.

It's tough in a whole new way.

The U.S. study was carried out by professor Suniya Luthar from Arizona State University and assistant professor Lucia Ciciolla from Oklahoma State University. They interviewed more than 2,000 well-educated mothers with children ranging in age from babies to young adults.

They found that mothers of tweens reported feeling lonely, empty and dissatisfied, and were most likely to suffer depression. Even mothers of teens were happier.

For a lot of parents, it's a shock when their kids start rolling their eyes and behaving dismissively.

"Many mothers aren't aware that the big separation from offspring, the one that really hurts, doesn't occur when children leave the nest, but when they psychologically pull away from their mothers," Luthar tells NPR. "This is a time of psychological metamorphosis for both mother and child."

On top of that, a lot of women are entering perimenopause as their children are entering the tween years. That means they're having hormonal shifts as well.

Psychiatrist Dr. Louann Brizendine says for most women, estrogen and progesterone levels start dropping after the age of 42. That can cause women to feel less nurturing, and to become more agitated with themselves, their partners and their children.

Luthar points out that mothers of tweens don't always have the same support as they did when their children were younger. Those close-knit mothers groups might have drifted apart. Yet being able to talk to other parents who are going through the same thing is as important as ever.

“We are working on an intervention program to foster mutual supportiveness among mothers not just when they first transition to motherhood, but importantly, through these very challenging years around the transition to puberty," Luthar tells The Independent.

But there's some good news to come out of the study. Not only do mothers of teens feel happier than mothers of tweens, mothers of adults feel the happiest of all, reporting the lowest levels of stress and the most satisfaction with life. So that's something to look forward to.