If you were to take the Trier Social Stress Test (TSST), as nearly one hundred fifty pregnant women did in a study led by Sonja Entinger at the University of California at Irvine, you’d be led to a windowless room with a video camera and instruments that measure your vital signs. There, an assistant would ask you to sit and be hooked up to instruments that measure your vital signs. In the room you’d also find three men and women sitting at a table, waiting for you.

They are your interview panel.

Facing them belly-on, your instructions are to pretend that you’re applying for a job and must deliver a five-minute speech to convince them that you’re right for the position. Someone would say 1-2-3-GO, and you’d start babbling, hopefully coherently. If you have nothing more to say before your time is up, one of your interviewers will blandly instruct you to continue. Run out of words again and twenty seconds of eerie silence will fill the room. And when you’re finally done, you’ll be asked to do a bit of mental math — say, to count down, in increments of thirteen, from a large prime number like 54,499. Before and afterward the fifteen minute ordeal, a researcher will enter the room and hand you a swab to collect your saliva for testing.

Analyzing all the data from their study, including an analysis of body language and hormone levels of women who took the TSST, the UC Irvine researchers confirmed something remarkable: the further along a woman was in her pregnancy, the less stressful she found the stress test. Compared to their stress levels in second trimester (17 weeks), volunteers in their third trimester (31 weeks) had lower blood pressure, slower heartrates, and lesser spike in the hormone cortisol. Pregnant women also did not stress out as much as nonpregnant controls who took the same tests at the same time intervals. This was not the first study that found that pregnant women, especially those in third trimester, are calmer than nonpregnant women under the same (short and moderately stressful) circumstances. But it was the first time that the same women were tracked at different stages of gestation.

So what is that makes pregnant women more Zen as they approach their due date? The likely answer is that the body reduces the sensitivity of cortisol receptors, even though baseline levels of the stress hormone are higher. In other words, it takes more stress hormones than usual to get the nervous system all hot and bothered. At the same time, the placenta increases production of an enzyme that changes cortisol to an inactive form, meaning that less of the toxic stuff filters through to the baby. Near the end of pregnancy, probably to calm you down before labor and help you bond with the baby, your body also produces more of the nervous-system soothing hormones oxytocin and prolactin.

All this is good news for moms who are slammed with short-term mild to moderate stress late in their pregnancies.

But there’s an even bigger surprise to come out of this. You may think this is your body subconsciously protecting the baby at a time of stress. But it’s just as likely that it works the other way around: your baby protecting you (as well as herself), because her placenta is responsible for at least some of the stress-dampening response to cortisol. It’s a beautiful idea — mother and child soothing one another in the face of life’s assaults.