Original Article
The expression and adaptive significance of pregnancy-related nausea, vomiting, and aversions on Yasawa Island, Fiji

https://doi.org/10.1016/j.evolhumbehav.2014.09.005Get rights and content

Abstract

We report a study on nausea and vomiting of pregnancy (NVP) and pregnancy-related food aversions in a small-scale society from Yasawa Island, Fiji. Because NVP has rarely been studied quantitatively in small-scale populations, we begin with a detailed description of its expression among the women of Yasawa. We found that 66% of these women experience nausea and/or vomiting in tandem with the development of aversions to certain foods. This pattern of expression is similar to what has been documented for industrialized populations, and the prevalence of 66% is close to the industrialized mean prevalence of 69%. We then use the data from the women of Yasawa to evaluate the three main hypotheses that have been put forward to explain the evolution and ecological function of NVP. We show that food aversions of pregnancy focus preferentially on food types that are more likely to carry pathogens or contain chemical toxins. Such aversions do not focus on nutrient-dense foods or on frequently encountered foods. These findings are most consistent with the hypothesis that NVP, along with pregnancy-related aversions, evolved to motivate women to avoid exposure to diseases and other toxins when they are immune-compromised by pregnancy and during a critical period of embryo development. These findings contribute to a growing body of theoretical and empirical literature that suggests that NVP symptoms represent a series of adaptations rather than pathological responses to the physiological demands of pregnancy.

Introduction

Sometimes called “morning sickness” or “pregnancy sickness”, Nausea and Vomiting of Pregnancy (NVP) refers to a suite of symptoms that many women experience to varying degrees and in varying combinations during early pregnancy. These symptoms include nausea, gagging, retching, vomiting, dizziness, and fatigue (Firoz, Maltepe, & Einarson, 2010). Increased olfactory sensitivity (Nordin et al., 2007, Nordin et al., 2005) and the development of novel aversions to specific foods, social situations, and/or sexual behaviors (Fessler et al., 2005, Navarrete et al., 2007, Young and Pike, 2012) generally accompany NVP and appear to relate to it both temporally and functionally (Patil, Abrams, Steinmetz, & Young, 2012).

Currently, NVP is thought to affect approximately 60% of women during at least one of their pregnancies (Flaxman & Sherman, 2000). However, this prevalence estimate is based largely on Western populations (Patil et al., 2012) and therefore may be inaccurate. Western food production and health care systems differ substantially from those in many other places, and these factors may influence NVP expression. Significantly, the limited evidence from populations from a more diverse range of countries suggests that NVP prevalence varies cross-culturally (Einarson et al., 2013, Pepper and Roberts, 2006) and that some non-Western populations have lower rates than have been documented for Western nations (e.g. Anath and Rath, 1993, Christian et al., 1998).

NVP is puzzling from an evolutionary perspective (Fessler, 2002b, Flaxman and Sherman, 2000) because the expression of the appetite-suppressing features of NVP in early pregnancy (e.g. nausea, vomiting, and food aversion) limits maternal and fetal access to energy and other nutrients that promote fetal growth (e.g. Latva-Pukkila et al., 2010, Lee et al., 2004). Despite these costs, however, such symptoms are associated with reduced risk of spontaneous abortion (Forbes, 2002, Huxley, 2000), and increases in NVP severity correlate with improved outcomes in other measures of fetal survivorship and in infant and young child health (e.g. Latva-Pukkila et al., 2010, Nulman et al., 2009, Weigel and Weigel, 1989). This counter-intuitive pattern raises two important questions. First, is NVP a feature of most healthy, non-abortive human pregnancies regardless of ecological context or is it an anomaly of industrial socioeconomic systems? Second, if NVP affects most populations, why might it have a positive rather than a negative impact on pregnancy outcome?

Here, we report a study designed to address these questions. The study focused on an indigenous population of small-scale fisher-farmers from Yasawa Island, Fiji. In the first section of the study, we used recall-based interview data from women of Yasawa Island to generate summary statistics regarding the expression of NVP and related symptoms in this population, and then compared these statistics to those documented for other populations. In the second part of the study, we used the interview data to test the three main hypotheses that have been proposed to explain why NVP might have evolved despite its costs. The study, which is among the first to systematically assess NVP in a small-scale society, supports the hypothesis that NVP is a feature of the majority of healthy human pregnancies, and suggests that it evolved to protect the developing fetus from exposure to pathogens and chemical toxins.

Section snippets

Data collection

The data were collected in 2005–2006 as part of an on-going project led by one of us (JH) on the lifeways, psychology, culture, and evolutionary ecology of a population indigenous to Yasawa Island, Fiji.

Yasawa, situated on the northwestern edge of the Fijian archipelago, has a seasonal climate, featuring a hot, wet season, and a relatively mild, dry season. The island is home to ~ 900 people who live in six villages. The majority of Yasawans are subsistence-level fisher-horticulturalists (

Part I: expression of NVP on Yasawa, and comparison to global sample

The first part of this study had two aims. The first was to diagnose and characterize NVP and related symptoms among the women of Yasawa Island. The second was to compare the proportion of women from Yasawa that experience NVP to NVP prevalence in other populations from around the globe.

We describe expression of NVP on Yasawa in detail because, although ethnographers have previously reported anecdotes of pregnant women in small-scale societies experiencing nausea and vomiting, these reports do

Part II: testing evolutionary hypotheses for NVP using data from Yasawa Islanders

In the second part of our study, we used data from Yasawa Islanders regarding pregnancy-related aversions and non-pregnancy household diet to test a series of hypotheses regarding why NVP and related symptoms occur in contemporary populations.

Discussion and conclusions

In the first part of our study, we found that NVP characterizes the pregnancies of the majority of Yasawan women, and that both the expression and prevalence of NVP among Yasawan women are comparable to what has been documented for large-scale, industrial populations. Unexpectedly, we also found that, during pregnancy, many Yasawan women develop aversions to the smell of their husbands. In the second part, using Yasawa Islander data to test three evolutionary hypotheses for NVP, we found that

Supplementary Materials

The following is the Supplementary data to this article.

Supplementary material

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    We gratefully acknowledge the contributions of a number of organizations that supported this research. LM receives support from the Social Sciences and Humanities Research Council of Canada (SSHRC) and Simon Fraser University (SFU). MC is funded by the Canada Research Chairs Program (CRC), the Canada Foundation for Innovation, the British Columbia Knowledge Development Fund, SSHRC, and SFU. JH is supported by SSHRC, CRC, the Canadian Institute for Advanced Research and New York University’s Stern School of Business.

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