Post by Lani Cupo
The brain changes during pregnancy
Many people know of the unusual cravings associated with pregnancy. But how does pregnancy impact the nervous system to such an extent that dill pickle ice cream can change from disgusting to delicious? Or to the extent that memory function is impacted? Many neural and cognitive systems are affected during pregnancy, leading to some of these phenomena such as changes in smell, food cravings, and cognitive difficulties. While some of the neural mechanisms underlying these changes are understood to some degree, many remain unclear.
Perceptual changes associated with pregnancy
Olfaction (smell)
Some people report changes to smell before even confirming a pregnancy, and over two-thirds of pregnant people report enhanced sensitivity to smell. Additionally, up to 14% reported phantom smells during pregnancy. This increased sensitivity can be associated with morning sickness, most common during the first trimester of pregnancy. When surveyed retroactively, 75% of 500 women recalled that smells were less pleasant during pregnancy, including cigarettes, coffee, meat, food (in general), diesel exhaust, and sweat, with similar reports from lab research. A few scents (clove, musk, perfume, fruits, flowers, and woodlands) have been rated as more pleasant
To date, no study has definitively established the neuroscience underlying changes in olfactory perception. One hypothesis takes an evolutionary perspective, proposing that enhanced smell perception provides protection for the fetus by increasing the mother’s disgust response during a period of immunosuppression. There is some evidence to support the hypothesis that nausea and vomiting early in pregnancy is linked to maternal avoidance of potential food-borne pathogens as the foods that are generally averse are more likely to carry pathogens, such as meats, dairy, eggs, and bitter vegetables.
Gustatory (taste)
Perhaps more famous than olfactory changes during pregnancy are changes to gustation, where pregnant people crave specific foods. Most frequent cravings include sweet foods such as chocolate, milk, and fruit, however in a report on preferred flavors, the majority of women reported sour tastes, followed by salty. Along with cravings, there are several common food aversions, including meat, eggs, oregano, and bitter foods. Along with pregnancy, food cravings are most commonly experienced during perimenstrum (PMS – the ~8 days before onset of menstruation). Despite the commonality of food cravings in pregnancy, for some women unrestrained eating during pregnancy can contribute to excessive gestational weight gain, which can adversely affect both the mother and fetus. Of interest, while food cravings are reported globally, the type of food desired often changes regionally, with high rates of chocolate craving reported in North America (in all populations). In a survey of pregnant people, 50-90% of individuals experience cravings, most of whom had a prior history of cravings.
Similar to olfaction, the scientific literature offers no definitive explanations for changes to gustation. At present, there are several proposed hypotheses for food cravings. One suggests that changes to gustation reflect changes in nutritional requirements - as sodium requirements increase during pregnancy, this could be reflected in the increased affinity for salt. However, the nutritional demands of pregnancy include micronutrients, iron, and magnesium, and rarely do pregnant people crave foods rich with these nutrients, such as dark leafy greens or legumes. Several proposed hypotheses closely resemble the postulated hypotheses for perimenstrual food cravings. An early hypothesis suggested estrogen or progesterone levels may underlie the changes, however, there is little empirical evidence to support this hypothesis. One theory from the mid-nineties suggests endogenous opioid peptides (EOPs) may play a role in inducing food cravings, substantiated by an association between EOPs and food intake.
In addition to the possibility of physiological changes regulating cravings, there could be a potential psychosocial component of food cravings. One framework suggests that food cravings may arise from the tension between a desire to indulge in foods and avoidance, with cultural events removing the expectation of abstinence (such as PMS or pregnancy) leading to overindulgence. In this model, pregnancy (or PMS) is not seen as a cause of craving, but an allowance to acknowledge such desires.
Memory changes associated with pregnancy
50-80% of pregnant people report cognitive difficulties, with memory problems being the most common. However, cognitive changes during pregnancy receive less attention. One reason for this is the fact that while pregnant people subjectively report more memory challenges with everyday memory tasks around the home, they often perform as well as non-pregnant control participants on laboratory tasks. The research suggests pregnant people may have greater memory difficulty when more is demanded of their attention. The two types of memory they struggle with most include episodic prospective memory (remember to call your mother in two days) and habitual prospective memory (tasks that must be performed regularly, like attending appointments). While more research is required to establish the underlying cause of memory issues, there are a couple of hypotheses regarding the link. One potential explanation could be that increased cortisol or reduced estrogen leads to a negative effect on memory. However, aside from physiological changes, pregnancy is a time defined by many behavioral changes such as sleep disturbance and increased novel demands preparing for the baby. Along with disadvantages to memory, however, there are also reports of improved memory for faces. This improvement is speculated to be related to high levels of progesterone present during pregnancy.
What’s next?
Despite evidence from both surveys and the laboratory establishing cognitive and perceptual changes during pregnancy, there is little research into the biological changes underlying these alterations. We have a preliminary understanding of why smell, taste, or memory might be affected. Many hypotheses exist, however, more research is required to confirm some of these hypotheses and to understand the neural mechanisms underlying these changes.
