We all know that having kids changes your life completely. But now, researchers have discovered that co-parenting can also drastically change parents’ immune systems. You know that old adage, people start to resemble their pets? Well, as it turns out, cohabiting and co-parenting can also change the actual cells that make up your immune system. Eventually, the composition of co-parents’ immune cells start to resemble one another. This is so cool and weird.

Researchers at the Translational Immunology Laboratory at VIB in Belgium studied the changes to parents’ immune systems in 2016. Their findings are fascinating.

Adrian Liston was part of the team that documented these immunological changes. He says that eventually, “co-parents end up with more in common immunologically than identical twins.” The changes you experience after having children, and specifically while co-parenting, don’t just affect your day-to-day life. These changes happen at a cellular level and completely change the way your natural defense system works.

A lot about why this happens is still being researched. But, there are a few factors that likely have an affect on it. Changes to your behaviors and habits is a big one.

When couples start living together, it’s not uncommon that their daily habits and behaviors start to resemble one another. You might exercise together, sleep and wake together, or make changes to your health together. The same thing happens when you have kids, but on a much grander scale. On it’s face, it might not seem like much. But the researchers say these daily changes shape your immune system more than anything else.

The two things parents have too much and not enough of: stress, and sleep.

Parents know how much stress and lack of sleep can wreck your immune system. Turns out, they can actually contribute to the cellular changes we’ve talked about. As part of the study, Adrian Lisbon’s team tracked people in Belgium who got sick when sewage ended up in their drinking water. The people who had generally low levels of stress recovered much faster than people who reported high stress in their lives. Some of the stressed out people suffered long-term illnesses attributed to the exposure to sewage, like irritable bowel syndrome.

Obviously, there are lots of benefits to having kids, or else people wouldn’t keep having them! But if you’ve ever wondered why your body reacts differently to illness after your kids were born, or why you seem to mirror your partners’ reactions to certain environmental changes, the adorable little answers might be staring you right in the face.

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