Mothers-to-be may be sick of “morning sickness”, but does this symptom of pregnancy serve a purpose?

Pregnancy, a beautiful time in any woman’s life when she witnesses her child growing inside her, feels her baby kick for the first time, and spends a great deal time vomiting into her toilet. Rather misleadingly termed “morning sickness”, nausea and vomiting during pregnancy (or NVP for short) is experienced by around 70% of expectant mothers during their first trimester and is rarely confined just to the first half of the day. NVP can begin as early as 5 weeks into a pregnancy, peaks between weeks 8 and 12, and generally continues up until about week 20. But what is the point of NVP I hear you suffering mothers-to-be cry? Is there a reason for this less than appealing part of pregnancy or is it just an unwanted side effect of this miraculous event?

Despite the difficulties and unpleasantness NVP can bring the mother, mild to moderate forms of NVP have been widely associated with favourable outcomes for her baby. Reductions in the risk of preterm delivery, low birth weight and miscarriage have all been shown to accompany NVP and suggest this condition may in fact possess an important function in pregnancy. It should be noted that the nausea and vomiting discussed here does not include the pathological condition hyperemesis gravidarum, which occurs in approximately 1% of mothers-to-be and can lead to serious complications if left untreated.

There are a number of theories which may help explain why NVP has evolved as a part of pregnancy. The first of these sees NVP as a method of “communication” to a woman’s partner, alerting them to the pregnancy and the need to modify their behaviour accordingly. This would lead to a reduction in their desire to have sexual intercourse, instead providing more protection and an increased food supply to the expectant mother.

While it may sound like an attractive idea to have our partners evolutionarily programmed to wait on us hand and foot during pregnancy, the “communication” theory seems unlikely. Firstly, the peak of NVP occurs later than the cessation of periods, an equally clear and a less unpleasant signal of pregnancy. NVP would therefore be superfluous, meaning it would be eliminated through natural selection. Secondly, there is no evidence to suggest sexual intercourse is detrimental to pregnancy and so no need to reduce its desirability.

An alternative hypothesis is that NVP is a side effect, or “by-product”, of the internal conflict which occurs between the expectant mother and her foetus. This is not an aggressive or violent form of conflict of course, but rather a competition for the mother’s limited resources. Pregnancy, childbirth and parenthood are all costly investments for a mother, and while taking more of her nutritional intake allows the foetus to maximise its fitness, this act also reduces the nutrition available to the mother and consequently lowers her fitness. Such a tussle for resources is bound to result in visible side effects, hence the presence of NVP.

Similarly to the “communication” hypothesis, this “by-product” theory has a number of flaws. For example, if NVP were a sign of foetal fitness, the presence of NVP should denote a successful pregnancy. However, NVP does not occur in all viable pregnancies, nor does its presence always result in positive pregnancy outcomes. In addition, this theory suggests that NVP symptoms should occur later during pregnancy when the foetus is larger and therefore requires more resources which, as previously discussed, is not the case.

The final and most widely favoured theory for the function of NVP is the “mother and embryo protection” hypothesis. This states that NVP acts to reduce an expectant mother’s intake of agents which could harm her pregnancy (known as teratogens), including caffeine, alcohol and tobacco. It also removes any dietary toxins or food-borne teratogens which are ingested by the mother-to-be before they reach the baby and, as a consequence, teaches her to avoid these foods. Hence, the well-known “food aversions” experienced by many pregnant women. By the same method, NVP and food aversions protect the expectant mother from foods that may contain pathogenic microorganisms that could make her ill. This is particularly important during pregnancy as a woman’s immune system is lowered during this period to prevent her body rejecting the embryo, which appears to her body as a foreign tissue.

Many of the features of NVP and pregnancy support the “mother and embryo protection” theory. To begin with, NVP symptoms usually occur in the first trimester, at the same time that the expectant mother and embryo are most immunologically vulnerable and therefore need increased protection from toxins and teratogens found in food. In accordance with the “protection” hypothesis, food aversions also tend to be greatest during the first trimester and the types of food that pregnant women tend to find aversive are those most likely to contain pathogenic microorganisms or teratogens, such as meat, caffeinated drinks and alcohol.

Over the years, a number of theories have been put forward, aiming to provide a reason for the characteristic “morning sickness”, or NVP, experienced by the majority of expectant mothers in their first trimester. Whether the front-running “mother and embryo protection” hypothesis is true or if another explanation exists, NVP certainly does appear to possess a function, being widely associated with positive pregnancy outcomes. While this is unlikely to make the experience of NVP a pleasant one, hopefully such knowledge will provide at least some comfort to all the mothers-to-be out there currently well acquainted with their toilet bowls.

Post by: Megan Freeman @Meg_an12