My friend recently asked me: “Why have I become so forgetful since I became pregnant?” I told her I didn’t know, but that I’d look into it. She then followed with: “I was going to ask you to explain something else to me, but I totally forgot what it was.”
It’s a common claim that pregnancy makes you forgetful. But does “pregnancy brain” actually exist? There’s no doubt that many changes happen to a woman’s body during pregnancy, but how do these changes affect – or originate in – the brain? To answer my friend’s question, and in an effort to address whatever else she was forgetting at the time, here is part one of my expectant mother’s guide to the crazy neuroscience of pregnancy.
More than half – perhaps even up to 90 per cent – of pregnant women experience nausea or vomiting to some degree, particularly in the morning. Thrust into the limelight as a result of the Duchess of Cambridge’s hospitalisations, around 1 per cent of pregnant women experience more severe, prolonged morning sickness called hyperemesis gravidarum, which can result in dehydration and weight loss, and may require medical attention. For most women, morning sickness goes away after 18 weeks.
The cause of morning sickness isn’t entirely clear. The most popular theory is that morning sickness is the body’s reaction to the increase in the pregnancy hormone, human chorionic gonadotropic (hCG).
Studies have shown a temporal relationship between hCG and morning sickness, meaning that levels of hCG in the bloodstream and frequency of vomiting appear to peak at the same time. The correlation is interesting, but it doesn’t explain why morning sickness happens.
Psst: So, you're growing a human inside you. Listen to Hello Bump. Post continues after audio.
We do know that the first three months (the first trimester) is an important time in foetal development. The central nervous system forms during this time, and this delicate process is easily disrupted by toxins circulating in the mother’s bloodstream. A more recent theory suggests that vomiting during early pregnancy serves a beneficial function by ridding the body of food that may unsettle this important developmental stage.
Vomiting is controlled by an area in the hindbrain called the area postrema. Importantly, the area postrema lacks a blood-brain barrier, which means it can detect toxins in the bloodstream and cerebrospinal fluid. Research has shown that the area postrema has receptors for hCG, which may explain why it’s particularly sensitive during pregnancy.
This “toxin theory” is supported by several pieces of evidence, including the fact that morning sickness is more common in societies with “risky foods”, that it only occurs in humans (we have very broad diets, after all), and that more severe morning sickness is associated with lower rates of miscarriage.