Hum. Reprod. Advance Access originally published online on September 3, 2007
Human Reproduction 2007 22(11):3009-3012; doi:10.1093/humrep/dem274
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Psychological distress during early gestation and offspring sex ratio
1 Department of General Practice, Institute of Public Health, University of Aarhus, 8000 Aarhus C, Denmark 2 Department of Paediatrics, Aarhus University Hospital, Skejby, 8200 Aarhus N, Denmark 3 Department of Obstetrics and Gynecology, Hvidovre University Hospital, 2650 Hvidovre, Denmark 4 School of Public Health University of California, Berkeley, CA 94720, USA 5 Department of Obstetrics and Gynecology, Rigshospitalet, 2100 Copenhagen, Denmark
6 Correspondence address. Tel: +45-89426059; Fax: +45-86124788; E-mail: co{at}alm.au.dk
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Abstract |
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BACKGROUND: Exposure to severe stress in early pregnancy is associated with a lower male to female ratio (sex ratio), but whether more moderate levels of psychological discomfort have the same kind of effect is unknown. In a population based follow-up study, we aimed to test whether psychological distress was associated with the sex ratio in the offspring.
METHODS: From 1989 to 1992, a cohort of 8719 Danish-speaking pregnant women were followed until delivery. Questionnaires were administered to the women in early pregnancy and 6629 (76%) completed the 30-item version of the General Health Questionnaire (GHQ).
RESULTS: We found an overall male to female ratio (sex ratio) of 1.03. There was an inverse dose response association (test for trend P < 0.01) between GHQ score and sex ratio. Each 5-point increase in the GHQ score was associated with a decreasing odds of having a boy [Odds ratio (OR) = 0.93, 95% CI 0.89–0.98]. Mothers scoring in the upper quartile of the GHQ had 47% boys as compared with 52% in the undistressed groups (Risk difference =4.8%, 95% CI 1.9–7.7%) resulting in a significantly lower sex ratio of 0.85 compared with 1.07 (OR 0.82, 95% CI 0.72–0.94).
CONCLUSIONS: Our results suggest that not only severe stress, but also more moderate and common levels of psychological distress, may decrease the sex ratio in the offspring. Stress during pregnancy is a likely candidate involved in the decreasing sex ratio observed in many countries.
Key words: psychological distress/stress/pregnancy/sex ratio/GHQ
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Introduction |
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In developed countries
5% more boys than girls are delivered (Biggar et al., 1999
). However, several recent studies indicate that the male to female ratio (the sex ratio) is declining (Moller, 1996; van der Pal-de Bruin et al., 1997; Marcus et al., 1998). This decline is of public health interest because sex ratio can be regarded as a sentinel health indicator (Davis et al., 1998).
The reason for this development is unknown. Smoking around the time of conception has been proposed as a possible cause (Fukuda et al., 2002), but this has not been confirmed in larger datasets (Mills et al., 2003; Obel et al., 2003) and seems an unlikely explanation since the prevalence of smoking during pregnancy is declining (Wisborg et al., 1998). Ecologic studies have shown that environmental exposures to flooding, earthquake or war were followed by a reduced sex ratio (Lyster, 1974; Fukuda et al., 1998; Graffelman and Hoekstra, 2000). In line with this, recent reports found a tendency to a lower sex ratio in the offspring of women, who were pregnant around the terrorist attack of September 11, 2001 (Catalano et al., 2005a, 2006). A common link between these studies could be exposure to psychological stress. The strongest support for this hypothesis is a Danish registry-based study, which found fewer boys in the offspring of women exposed to severe stress around or just prior to conception (Hansen et al., 1999). However, only very rare and extreme stress exposures were considered. To explain the secular trend of decreasing male to female ratio, more moderate stress exposure needs to be considered.
The effect of such levels of psychological distress has so far only been studied indirectly in an ecological design, e.g. following the use of antidepressants and anxiolytic drugs (Catalano et al., 2005b). In the present report, we examine the association between more moderate and common levels of maternal psychological distress and the sex ratio in the offspring in a follow-up design.
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Materials and Methods |
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Between August 1, 1989 and September 30th, 1991, a population-based cohort of pregnant women was established at the Department of Obstetrics, Aarhus University Hospital, Denmark. Details about the data collection have previously been published (Hedegaard et al., 1993
).
Briefly, the study was based on information from women who during pregnancy attended the Hospital for routine antenatal care. The women were sent by mail self-administered questionnaires twice during pregnancy, prior to their routine antenatal visits at approximately the 16th and the 30th week of gestation. The present paper is based only on exposure-information related to the first trimester collected at the visit around 16 gestational weeks of pregnancy by two questionnaires. One was a brief questionnaire designed to provide details for the women's medical record including information on medical history, demographic characteristics and smoking and alcohol habits, and the other was a more extensive research questionnaire including the 30-item version of the General Health Questionnaire (GHQ) (Goldberg, 1972).
We used the GHQ (Goldberg, 1972), which was developed as a screening instrument for general populations. The score can be interpreted to indicate the severity of psychological disturbance. The content and construct validity have previously been tested in a Danish population (Bech, 1987), and in the present study, the reliability of the GHQ was found to be high (Crohnbach's alfa >0.90). The 30 items asked about different aspects of psychological distress experienced within the previous month, i.e. approximately during the third to fourth gestational month. The possible answers were (i) ‘not at all’, (ii) ‘not more than usual’, (iii) ‘somewhat more than usual’ or (iv) ‘much more than usual’. On this basis, the women were asked to indicate the level of distress compared with a personal (internal) reference. Hence, women who were more chronically distressed were unexpected to obtain the highest scores.
Each item in the GHQ was dichotomized with zero points for the two lower categories of answers on a given item, and 1 point otherwise, as recommended by Goldberg et al. (1997). The sum across the 30 items was calculated, resulting in a score with a potential value between 0 and 30. The scores of the women in our study ranged from 0 to 29. The median value was 2, the upper quartile was a score above 5 and the upper centile was a score above 9. Because of the skewed distribution, GHQ was analysed as a categorical variable. To investigate a possible dose response association, we categorized the GHQ scores a priori in equal steps of the score (0, 1–5, 6–10, 11–15, 16–20, 21–25, 26–30).
Women, who were unable to read Danish were excluded from the study (n = 470). Furthermore, women with twin pregnancies were excluded (n = 123), leaving 8719 women eligible for the study. Of these, 6643 women (76.2%) completed the questionnaire. The sex of the newborn in 14 deliveries was not reported, leaving 6629 cases for the analyses. Women were asked to record the date on which they completed the questionnaires.
After delivery, the attending midwife recorded the information about the delivery and the newborn on a structured coding sheet. This information included weight, length, head circumference, APGAR score and sex of the child. A research midwife reviewed the medical records, validated and completed the coding sheet by adding information on the post-partum period.
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Statistical analysis |
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Data analyses were performed in SPSS®, version 11.0. Chi-squared test for independence within contingency tables and Chi-squared test for trend were used for categorical data. Statistical significance was defined as a two-sided P-value of <0.05. Multivariate logistic regression analyses were carried out to compare the sex ratios according to the level of distress and adjust for potential confounding factors. Potential confounders were selected based on previous reports. Factors changing the estimate by 10% or more were included in the final model (Greenland, 1989
). Risk differences (RD) and odds ratios (OR) were calculated for unadjusted analyses and OR's for adjusted analyses. Both were provided with 95% confidence intervals (CI). Women who reported no distress (GHQ score of zero) were used as the reference group. |
Results |
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Among participants, 3364 (50.7%) women delivered a boy, whereas 3265 (49.3%) women delivered a girl resulting in a sex ratio of 1.03. The sex ratio in the entire cohort was 1.05. The sex ratio among non-participants was higher, but not statistically different from that of the participants (OR = 1.07, 95% CI 0.96–1.18).
Table 1 describes the sex ratio of the infants in relation to obstetric, behavioural and demographic factors. Only maternal height was significantly associated with the sex of the newborn. We found a slightly lower sex ratio among the youngest and the oldest mothers and fathers. Women who had received infertility treatment also had a reduced sex ratio.
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The association between psychological distress and the sex ratio in the offspring is shown in Table 2. We found a decreasing proportion of boys with higher levels of psychological distress. The sex ratio in the reference group was 1.07 compared with a ratio of 0.67 in women who reported the highest level of psychological distress. The test for trend suggested a dose-response relationship (P < 0.001), with each 5-point increase in the GHQ score being associated with a decreasing chance of having a boy (OR = 0.93, 95% CI 0.89–0.98). Women in the upperquartile of GHQ scores, i.e., those who scored greater than 5, had 47% boys (resulting in a sex ratio of 0.85) and were thus 5% less likely to have a boy (RD = 4.8%, 95% CI: 1.9–7.7%) as compared with those who reported no symptoms of distress at all. They had significant lower odds of delivering a boy (OR 0.82, 95% CI 0.72–0.94).
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The association between the GHQ score and the sex of the baby could potentially be confounded. In order to investigate this possibility, we conducted a series of multivariate logistic regression analyses with the sex of the baby as the outcome. In Table 2, we present data adjusted for all the covariates in Table 1. However, none of the potential confounders were able to change the crude OR estimates by 10% or more. The unadjusted data (only containing the GHQ score) was therefore regarded as the most valid.
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Discussion |
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We found fewer boys among the offspring of women who reported moderate to high levels of psychological distress. Among the children of women in the upper quartile of GHQ scores, we found 47% boys compared with 52% in the unstressed women and a significant dose response association.
Our findings are in accordance with the findings of Hansen et al. (1999), who showed a reduced sex ratio among women who experienced trauma (i.e. death or admission to hospital of an older child due to cancer, or death or admission to hospital of the spouse due to cancer or myocardial infarction) during the 12 months prior to pregnancy or during the first trimester. They found the lowest sex ratio in the newborns of women exposed to stress during the first trimester. Women, who experienced the death of a spouse or an older child, gave birth to babies with a sex ratio of 0.65. This is comparable to the ratio found among the few women in the present study (1%), who reported very high levels of distress. Our findings also agree with a number of ecological studies that have found a lower sex ratio in populations exposed to severe stress (Lyster, 1974; Fukuda et al., 1998; Graffelman and Hoekstra, 2000; Catalano et al., 2006).
The participation rate in the present study was 76%. The sex ratio in newborns of women who declined participation was slightly higher than among the responders. Less than 10% of all women had prenatal chromosome testing of their fetus. Results of these tests were usually unavailable until the questionnaire had been completed. The women were therefore unaware of the sex of the fetus at the time of accepting or declining participation and we found no reason to suspect that selection bias can explain our findings.
We tested potential confounding effects from a number of other factors suspected to influence the sex ratio. However, none of these factors affected the association between the GHQ score in early pregnancy and the sex ratio in the offspring. We are unaware of other potential confounders that may have explained our findings.
Trivers and Willard's hypothesis that maternal conditions such as stress could influence the sex distribution of the offspring (Trivers and Willard, 1973) has found support in a number of animal experiments (Pratt and Lisk, 1989; Bacon and McClintock, 1999). They suggest that the effect may be mediated by corticosteroids (Pratt and Lisk, 1990). Psychological distress may activate the maternal hypothalamic–pituitary–adrenal axis and expose the fetus to high levels of cortisol. The GHQ was used as a measure of psychological distress in the present study and the instrument has previously been validated in pregnant women (Sharp, 1988). GHQ is a reliable measure of psychological discomfort and screens for minor psychiatric disorders (Goldberg et al., 1997). However, documentation of the association of GHQ markers with biological measures of human stress such as serum corticosteroid levels is quite limited (Aardal-Eriksson et al., 1999).
As our measures of psychological distress were not defined by a specific event, we were unable to define an exact time window of exposure. We collected a measure of psychological distress related to the third to fourth month of pregnancy. This measure may be a weak indicator of peaks of stress during the entire first trimester of pregnancy and we cannot determine if there is a critical period within the first trimester. The fewer boys born to women in the groups experiencing higher stress levels may be due to a differential susceptibility to miscarriage. Unfortunately, we had no data on the stress levels of women whose pregnancies ended in miscarriage or on the chromosomes of the spontaneously aborted fetuses, but it has been reported that spontaneously aborted fetuses have a higher male to female ratio than newborns (Byrne and Warburton, 1987). Even in early pregnancy, a male fetus has a faster growth rate (Henriksen et al., 1995) which may explain the differential vulnerability. We measured the distress level in first trimester but this may also reflect stress levels around the time of conception. It has recently been proposed that non-optimal conditions at this time for male zygotes could cause non-optimal maturation and suboptimal implantation resulting in a higher risk of early loss of male fetuses (Jongbloet, 2004).
In conclusion, our findings support animal evidence by showing that the male to female ratio, also among humans may be decreased by stress during the first trimester, not only at extreme levels, but at levels that affect a substantial part of the pregnant population. Our findings suggest that stress may contribute to the shift in human sex ratio observed in developed countries over the last decades (Moller, 1996; van der Pal-de Bruin et al., 1997; Marcus et al., 1998).
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Funding |
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This project was funded by the Danish Medical Research Council.
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Submitted on April 2, 2007; resubmitted on June 11, 2007; accepted on July 17, 2007.
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