Now at the start of third trimester, I find it inconceivable that there are women at this stage — and beyond — who still do not know they’re pregnant.

Here are the astonishing statistics: 1 in 455 women doesn’t know she’s pregnant until after week twenty, and 1 in 2,500 is oblivious until she actually goes into labor. The latter are known to give birth, without medical assistance and in agonizing pain, in Walmart bathrooms and at proms, in dorm rooms and in their own bathrooms. They had no idea they were pregnant because they had irregular periods, have been on birth control pills, are in perimenopause, have had menstrual-like bleeding, and/or are overweight and less sensitive to weight gain.

But I know what you’re thinking because I’ve thought it too: it’s denial. On some level the ladies must’ve known they were pregnant but couldn’t deal with the reality.

Yet the more I explore the origins of cryptic pregnancy, as the condition is clinically called, I realize that denial or mental illness doesn’t explain most of the cases. Only a minority of cryptic pregnancy cases has been attributed to personality disturbances (8 percent) or schizophrenia (5 percent). It appears that most of these women are perfectly sane, educated, and in stable relationships. Quite simply, they do not know they’re pregnant because they have no symptoms — no weight gain, no nausea, and little to no abdominal swelling. Or the symptoms are so subtle as to be easily mistaken for something else.

Every pregnancy is a tug-of-war of resources between Mom and fetus. Each has her self-interest in mind. Most of the time the tug-of-war ends up in a happy equilibrium. Mom provides enough nutrients, but not too much too handicap herself. But sometimes Mom gets more rope….at the expense of the fetus.

According to evolutionary psychologist Marco Del Giudice this might happen in a few ways. For one, the fetus might not be putting out enough signals that it exists and needs resources. One way fetuses announce their existence is through HCG, the hormone that makes a home pregnancy test turn positive. In many cases, the higher the HCG, the more severe the morning sickness and other symptoms. A baby that produces a scant amount amounts of HCG might go “under the radar,” failing the pregnancy test and going unnoticed by the mother — physiologically and psychologically. This would mean the baby gets fewer resources than she otherwise would. The lack of HCG signaling in cryptic pregnancies explains why these babies are so often born preterm, underweight, and small for their gestational age. They didn’t ask for more resources from Mom, and they didn’t get any.

There are a few reasons why a baby wouldn’t produce enough HCG. One is chromosomal anomalies; that is, the fetus has a birth defect and is in danger of miscarrying. It’s also possible that an otherwise healthy fetus simply puts out low quantities of the hormone due to a genetic quirk.

Or, here’s an interesting theory: Maybe Mom has stress and relationship problems. In this case, biologically speaking, it may be in the fetus’s best interest for the mother to be completely oblivious to the fact that she’s carrying to prevent being rejected and miscarried, which may happen when a woman is stressed. As Del Giudice points out, in our evolutionary past a woman who did not know she was pregnant and had few to no symptoms could conserve precious energy. She would be able to move freely and eat food of any kind, and as a result be better able to survive in the face of stresses and threats. In this case, babies may put out less HCG or stressed-out moms may unconsciously lower their sensitivity to the hormones.

Seen this way, cryptic pregnancy is an adaptive “emergency” mechanism — essentially, the fetus sensing a threat and striking a bargain with the mother by demanding little and laying low. When the normal stresses of pregnancy might otherwise trigger a miscarriage, this “stealth strategy” allows the fetus to survive.