Maternal longevity and the sex of offspring in pre-industrial Sweden

BACKGROUND: Helle et al. (2000. Sons reduced maternal longevity in preindustrial humans. Science, 296, 1085) argued that giving birth to sons reduced maternal longevity in pre-industrial societies due to higher physiological costs of bearing sons and the elevated testosterone levels observed in mothers carrying male foetuses. AIM: The present study examined this hypothesis using a more comprehensive dataset and evaluated the merits of the statistical approach used in previous studies to identify the cost of giving birth to sons in terms of maternal old-age longevity. SUBJECTS AND METHODS: The analysis in Helle et al. (2002. Sons reduced maternal longevity in preindustrial humans. Science 296, 1085) was extended by using a considerably larger dataset of pre-industrial Swedish women, and with careful consideration paid to methodological problems of sample selection and omitted variable bias. We argue that the previous literature has underestimated the difficulties in quantifying the trade-off between parity and longevity due to unobserved heterogeneity in health. However, under less restrictive assumptions, one can estimate the marginal impact of a son for a fixed family size. RESULTS: No evidence was found of a negative relative impact of sons. Neither was any evidence found in favour of the male-biased intra-household resource competition hypothesis proposed elsewhere in the literature, despite the poverty of the study population. These results are robust to a wide range of specifications tested. CONCLUSION: The failure to reproduce earlier findings and the fact that studies in this area of research seem to continue to yield conflicting results warrant much caution in discussing and evaluating results. It is likely that the negative effect of sons, if it existed, only manifested itself under conditions that are not yet fully understood. We also argue that the previous literature on this topic has not fully acknowledged the inference problems associated with omitted variable bias and sample selection.     

Maternal longevity and the sex of offspring in pre-industrial Sweden

Background: Helle et al. (2000. Sons reduced maternal longevity in preindustrial humans. Science, 296, 1085) argued that giving birth to sons reduced maternal longevity in pre-industrial societies due to higher physiological costs of bearing sons and the elevated testosterone levels observed in mothers carrying male foetuses.

Aim: The present study examined this hypothesis using a more comprehensive dataset and evaluated the merits of the statistical approach used in previous studies to identify the cost of giving birth to sons in terms of maternal old-age longevity.

Subjects and methods: The analysis in Helle et al. (2002. Sons reduced maternal longevity in preindustrial humans. Science 296, 1085) was extended by using a considerably larger dataset of pre-industrial Swedish women, and with careful consideration paid to methodological problems of sample selection and omitted variable bias. We argue that the previous literature has underestimated the difficulties in quantifying the trade-off between parity and longevity due to unobserved heterogeneity in health. However, under less restrictive assumptions, one can estimate the marginal impact of a son for a fixed family size.

Results: No evidence was found of a negative relative impact of sons. Neither was any evidence found in favour of the male-biased intra-household resource competition hypothesis proposed elsewhere in the literature, despite the poverty of the study population. These results are robust to a wide range of specifications tested.

Conclusion: The failure to reproduce earlier findings and the fact that studies in this area of research seem to continue to yield conflicting results warrant much caution in discussing and evaluating results. It is likely that the negative effect of sons, if it existed, only manifested itself under conditions that are not yet fully understood. We also argue that the previous literature on this topic has not fully acknowledged the inference problems associated with omitted variable bias and sample selection.     

Maternal exposure to influenza and cold in pregnancy and electrodermal activity in offspring: The Mauritius study

Data are in conflict concerning whether a mother's exposure to influenza in pregnancy gives rise to an increased probability that her offspring will develop schizophrenia. In Northern Hemisphere studies, exposure to influenza and cold tend to be confounded. The present study, carried out in Mauritius, examines the effect of maternal exposure to the virus and separately to cold on aspects of electrodermal activity that have been shown in other studies to be related to schizophrenia. The findings are that maternal exposure to influenza in the second and third trimesters gives rise to children who at the age of 3 years show electrodermal hyperresponsivity, whereas exposure to cold in the same periods gives rise to children who tend to be hyporesponsive. In both instances, exposure tends to produce lower levels of tonic activity than in those not exposed to the virus or to cold.     

Changes in sex hormones and offspring sex ratio following gasoline exposure in male rats

An experimental study was performed to determine the effect of gasoline inhalation on sex hormones and offspring sex ratio in male rats. Twenty two Sprague–Dawley adult male rats were divided into two groups. In the first group (n = 11), animals were exposed to gasoline vapor for 6 h daily for 30 consecutive days. The second group served as control without gasoline exposure. In the end of the study, sex hormones including total testosterone, luteinizing hormone and, follicle-stimulating hormone were determined using radioimmunoassay methods. Then, animals in both groups were mated with healthy unexposed female rats and sex of offspring was determined after birth. Results showed that the level of all sex hormones was significantly decreased in test group compared with control ones (P < 0.02). Offspring sex ratio was 0.39 (38/96) in the test group, significantly lower than 0.54 (60/101) in the control group (P = 0.03). The potential of manipulating offspring sex ratio through a simple, safe, and practical method would be an interesting subject. The results of the present study could provide a baseline for future research on this subject.     

Maternal tolerance achieved during pregnancy is transferred to the offspring via breast milk and persistently protects the offspring from allergic asthma

Background Maternal, more than paternal, asthma is a risk factor for the development of asthma in children. Recently, epidemiologic studies have shown that environmental exposures during pregnancy might influence the development of childhood asthma and allergies. Objective The aim of the present study was to investigate whether the induction of tolerance against a specific antigen during pregnancy prevents in the offspring the development of allergic asthma in response to this antigen. Methods Balb/c mice were orally tolerized with ovalbumin (OVA) during pregnancy. The offspring of tolerized and naïve mothers were immunized with OVA at 6 weeks and 4 months of age and analysed in our murine asthma model. Results While the offspring of naïve mice developed increased AHR, eosinophilic airway inflammation, T‐helper type 2 cytokine production and high serum IgE levels in response to OVA sensitization, the offspring of tolerized mice were almost completely protected from asthma, even when immunized as late as 4 months after birth. Breastfeeding was crucial for protection because tolerance was only observed when the offspring were nursed by their own mothers and not when nursed by naïve wet‐nurses. Allergen‐specific IgG₁ antibodies were exclusively increased in the breast milk of tolerant mothers and serum of protected pups, indirectly supporting its important role in tolerance transfer from the mother to the offspring. Sensitization of the F1 generation from OVA‐tolerized mothers with a heterologous allergen enhanced the immune response to this antigen. Conclusion Our results demonstrate that mucosal allergen contact during pregnancy modifies the asthma and allergy risk of the offspring mediated via breast milk. This observation may suggest that the time window for primary prevention strategies starts even before early childhood during pregnancy.     

Maternal exposure to influenza in pregnancy and electrodermal activity in offspring: A further study from Mauritius

This study examined the effect of mothers' exposure to the 1957 A2/Singapore influenza virus in the first, second, and third trimesters of pregnancy on the electrodermal activity of their 15-year-old offspring. The epidemic occurred during cold months of the year. The results show that maternal viral (and cold) exposure during all trimesters of pregnancy produced a lower frequency of nonspecific electrodermal responses and diminished responsivity to 75-db and 90-db tones as compared with the offspring of mothers who were not exposed. However, there was a critical effect of second trimester exposure as indicated by an increase in frequency of nonresponses to orienting tones. The findings are compared with those from an earlier study from Mauritius involving 3-year-old children in which the effects of exposure to the 1968 A2/Hong Kong virus were examined and in which the effect of cold exposure could be examined separately because viral exposure occurred during the hot months.     

Maternal depression and psychiatric outcomes in adolescent offspring: a 13-year longitudinal study

BACKGROUND: Maternal postnatal depression (PND) has been associated with adverse outcomes in young children, but an association with longer-term psychiatric disorder has not been demonstrated. We present the preliminary findings of a 13-year longitudinal study. METHODS: In the course of a prospective longitudinal study, we examined DSM-IV Axis I disorders in 13-year-old adolescents who had (n=53) or had not (n=41) been exposed to maternal PND. We also detailed the occurrence of depression in mothers throughout the 13-year follow-up period. RESULTS: Maternal PND was associated with higher rates of affective disorders in adolescent offspring. However, mothers who developed PND were also substantially more likely than those who did not to experience depression subsequently, a fact that contributed to the development of depressive disorder in offspring. Maternal PND was associated with increased risk for depression in adolescent offspring only if there had also been later episodes of maternal depression. In contrast, anxiety disorders in offspring were elevated in the maternal PND group regardless of the occurrence of subsequent maternal depression. LIMITATIONS: Due to the modest sample size and consequently limited power, findings must be regarded as preliminary. CONCLUSIONS: The particular association between early maternal depression and anxiety disorders in offspring was consistent with theories that emphasise the primacy of early environmental exposures. This position was not supported with respect to offspring depressive disorder, where overall duration of maternal depression was a significant factor. PND was associated with recurrent episodes of depression in the majority of cases, underlining the need for monitoring of this population beyond the postnatal period.     

Psychological distress during early gestation and offspring sex ratio

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 8,719 Danish-speaking pregnant women were followed until delivery. Questionnaires were administered to the women in early pregnancy and 6,629 (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.     

Maternal fish intake during pregnancy and atopy and asthma in infancy

BACKGROUND: There is growing evidence that n-3 fatty acids have anti-inflammatory properties and may modulate immune response. Dietary intake of these nutrients during pregnancy could play a role in the risk of asthma and atopy in the offspring. METHODS: Using data from a cohort of women (n=462) enrolled during pregnancy and whose offspring were followed up to 6 years, we evaluated the impact of fish consumption during pregnancy on the incidence of atopy and asthma. Dietary intake was assessed by food frequency questionnaire (42 items) applied by an interviewer. RESULTS: Thirty-four percent of infants had a medical diagnosis of eczema at age 1 year, 14.3% of the children were atopic [based on skin prick test (SPT) at 6 years], and 5.7% had atopic wheeze at age 6 years. After adjusting for potential confounding factors, fish intake during pregnancy was protective against the risk of eczema at age 1 year, a positive SPT for house dust mite at age 6 years and atopic wheeze at age 6 years [odds ratio (OR)=0.73 95% confidence interval (CI) 0.55-0.98, OR=0.68, 95% CI 0.46-1.01 and OR=0.55, 95% CI 0.31-0.96, respectively]. For an increase in fish intake from once per week to 2.5 times per week, the risk of eczema at age 1 year decreased by 37%, and the risk of positive SPT at age 6 years by 35%. Stratification by breastfeeding showed that fish intake was significantly related to a decrease risk in persistent wheeze among non-breastfed children (P for interaction<0.05). No protective effect was observed among breastfed children. CONCLUSION: Our data suggest a protective effect of fish intake during pregnancy on the risk of atopy-related outcomes.