The science behind nausea and vomiting of pregnancy

Nausea is the most common symptom experienced by women during pregnancy. 

For those unlucky enough to experience it, we feel your pain. We really do. It’s the pits, plain and simple.

Typically, nausea begins ~4th week of pregnancy and affects 4 in 5 women during the 1st trimester. If you do the math on that, you’ll see it’s about 80%. If this isn’t, or wasn’t, you - you’re one of the blessed few to escape it!

1 in 5 women will continue to experience nausea and/or vomiting during the 2nd trimester and only a few women will experience nausea and/or vomiting throughout their pregnancy.

In 1 in 200 women, the condition progresses to hyperemesis gravidarum (HG), which is characterised by prolonged and severe nausea and vomiting starting before the end of 22nd week of gestation. If left untreated, or if treatment is unsuccessful it can lead to dehydration, and weight loss with significant implications for mother and babies health. This is a serious condition that requires medical attention. You don’t muck around with this. If you do suffer from HG, you need to call in all the support you’ve got available to you.

The bacteria Helicobacter pylori is more commonly identified in the stomach of women with HG, than in women without. While Helicobacter pylori is unlikely to be the cause of HG, it may exacerbate the normal changes that occur with pregnancy and lead to more severe symptoms

The information provided below is relevant to mild nausea and/or vomiting only.

The cause/s of nausea and/or vomiting in pregnancy are still considered 'unknown', but there are a few things we do know. 

Most women experience nausea and/or vomiting during the 1st trimester, therefore the increased human chorionic gonadotropin (hCG) and other hormonal changes (estrogen and progesterone) occurring during this time have been implicated.

There are some factors that increase the risk of nausea and/or vomiting in early pregnancy including age, family history, ethnicity, smoking status, sex of baby and some dietary factors including saturated fat intake and alcohol consumption. Despite these, when something affects so many members of a population (80% during the 1st trimester, remember?), it is usually considered adaptive and therefore beneficial for the species. 

We know, we know. That seems crazy. Why would the universe conspire against us in such an awful way? We’re making new humans for goodness sake! But read on, this does get interesting.

So what could be the advantage of nausea and/or vomiting early in pregnancy?

Food intake is a risky business. It exposes us, and our developing baby to possible food-related dangers such as viral and bacterial infection, allergens and food intolerances. A key survival mechanism is identifying which foods are safe. We, and other animals, rely on our sensory system to identify safe food. Smell and taste are the initial gatekeepers, with nausea and vomiting the back up, if an offending food makes it past the sensory system. Emesis (vomiting) and diarrhoea is our body's way of making sure dangerous toxins do not stay inside us. A bit gross, but very clever!

A hypothesis for why we are turned off some (all?!) foods during the 1st trimester relates to this being the time when our fetus is most vulnerable to teratogenic agents. A teratogenic agent is a substance that interferes with the development of our fetus. Food is a potential source of teratogens. During the first trimester, all of our babies' organs develop, therefore exposure to teratogenic agents during this time, can have devastating consequences for their formation and lifelong health.

There is evidence that meat and animal products are the foods women develop the strongest aversions to early in pregnancy. Sound familiar?

We may even have aversions to meat consumption during the luteal phase of our menstrual cycle, which is the period after ovulation and when implantation and development of our uterus in preparation for pregnancy occurs. Have you noticed this before?

So, what to do with all this information?

Firstly, know that your body is wise. It knows what to do to protect you, and any small people you might have on board. Nausea and vomiting are (unfortunate) symptoms, with a significant purpose. They act as insurance for the next generation.

Secondly, know that your baby's energy and nutrient requirements are incredibly low during the first trimester. Limited food intake during this period is not going to have a significant impact. However, be sure to maintain adequate hydration. Water is your friend, when food is not.

For those with the opportunity to plan for their pregnancy, the 6-12 months prior to conception is an ideal time for both parents to optimise their health. Establishing a healthy eating regime, regular exercise and obtaining a healthy weight for you will be highly beneficial for pregnancy success.

Does changing the conversation on nausea and/or vomiting early in pregnancy, and understanding that it is a protective mechanism for our baby, make the discomfort easier to manage? Maybe, but probably not! So we've written another blog here to help you learn how to self manage your nausea symptoms for some relief.