Here are some astonishing statistics: Among pregnant women, 1 in 450 doesn't know her status until week 20 or later (more than halfway through the pregnancy), and 1 in 2,500 is oblivious until she actually goes into labor.
I know what you're thinking because I've thought it too: it's denial. On some level, these women must know they're pregnant but can't deal with the reality.
Yet when I explored the origins of cryptic pregnancy -- the clinical name for this condition -- I realized that denial or mental illness doesn't fully explain the phenomenon.
Only a minority of cryptic pregnancy cases has been attributed to personality disturbances (eight percent) or schizophrenia (five percent). It appears that most unexpectedly expectant mothers are perfectly sane and educated. 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. They may still have their periods, or have always had irregular periods. If they have symptoms, they're so subtle as to be easily mistaken for something else. Indigestion, perhaps.
There's a very real and perfectly valid reason for this, according to Marco Del Giudice, a cognitive scientist at the University of Turin. And it's linked to an expectant mother's stress-levels.
Pregnancy is often a tug-of-war between mother and fetus for the mother's limited resources. Most of the time it ends in a happy equilibrium -- the fetus takes nutrients from the mother, but not enough to cause harm. The mother holds back, but not enough to harm the fetus.
Sometimes, though, under some conditions, mom gets a lot of rope -- at the fetus's expense.
One way that embryos and fetuses make their demands known is by putting out a hormone called human chorionic gonadotropin (HCG). This is the hormone that makes a home pregnancy test turn positive. A baby that produces a scant amount amount of HCG might go "under the radar," failing the pregnancy test and going undetected by the mother.
When a fetus doesn't put out much HCG, he or she also gets fewer resources. This explains why these babies are so often small for their age and are born preterm and underweight.
There are a few factors that contribute to a fetus not producing enough of the pregnancy hormone:
1. Chromosomal anomalies: That is, the fetus has a birth defect.
2. Genes: It's also possible that an otherwise healthy fetus simply puts out low quantities of the hormone due to a genetic quirk.
3. Stress: According to one theory, a fetus could produce low levels of HCG because the mother is facing extraordinary stress. In this case, it's in the fetus's best interest for the mother to be happily oblivious to the fact that she's pregnant. Sometimes, the normal stresses of a pregnancy combined with other stresses in a mother's life can lead to miscarriage. But a 'stealth' pregnancy that uses few resources in a mother's body can actually increase the fetus's chance of survival.
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. This stealth strategy isn't helpful in good times (there are advantages to nausea and food aversions and, obviously, a full-size, full-term fetus). But in hard times (famine, war, loss of mother's partner or support network, etc.) an undemanding, under-the-radar pregnancy could be tremendously advantageous. Under these conditions, fetuses put out less HCG, or stressed-out moms may unconsciously lower their sensitivity to the hormone.
Seen this way, a cryptic pregnancy is an adaptive "emergency" mechanism -- essentially, the fetus sensing a threat and striking a bargain by demanding little and laying low to maximize its chances of being born.
Bottom line: A negative pregnancy test doesn't always mean you're not pregnant (even weeks or months after a possible conception).
*If you like this blog, click here for previous posts. If you wish, check out my new book,Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.
Follow Jena Pincott on Twitter: www.twitter.com/jenapincott