Psychological stress at work is a rising problem in Denmark. Nearly one third of the women reported in 2005 that they had difficulties completing their work tasks, and 17 % found that they had only limited or no influence on their work tasks. The corresponding numbers for 1987 were 18.3 % and 16 %, respectively. Work-related stress shortens the life expectancy and reduces the number of years without prolonged disease. For the society work-related stress amounts to more than 30,000 hospital admissions each year, half a million extra days on sick-leave for women, 500,000 contacts to general practitioners, 1600 early retirements for women, and an overuse of the health-care system. With the second highest employment rate in Europe for women - and many of them in the childbearing age - effects of psychological stress at work may extend beyond the exposed individual and affect pregnancy, birth and health of the child. Few studies on job stress relative to pregnancy have been carried out, but both animal and epidemiological studies have shown effect of exposure to stressful conditions during pregnancy and adverse effects on the offspring. The specific aims for the three studies included in this thesis were to investigate the association between maternal psychosocial job strain during pregnancy, measured as high demands and low control and the risk of: - Having a child born preterm or with low or high birth weight relative to gestational week (paper I + II) - Congenital malformations in offspring (paper III) - Asthma and atopic dermatitis in the children (paper IV). Furthermore, it was also the ambition to maximize and secure the quality of research and integrity of the data used by documenting the methods in a protocol that described the analyses before they were done and to keep transparency in the methods used following good epidemiological practices (GEP) for occupational and environmental epidemiological research. All analyses in this thesis are based on information and data from the Danish National Birth Cohort (DNBC). The cohort was established in 1994 and baseline information from the enrolled 100,418 pregnancies was gathered from 1996 to 2002. At their first antenatal visit pregnant women were invited to participate in the cohort by their general practitioner. To contribute the women had to be pregnant, have intentions of carrying the pregnancy to term, reside in Denmark and speak Danish sufficiently well to participate in telephone interviews. When enrolled the women were asked to participate in two telephone interviews during pregnancy at approximately 12-14 and 30-32 weeks of gestation and two after birth when the child was six and 18 months old and a follow-up questionnaire at age seven years. Exposure to work-related stress was assessed based on information from the first interview on two questions regarding job control and job demands. These questions were interpreted as dimensions of demands and control, and hereafter used as a proxy for the dimensions of the job strain model by Karasek. Based on their answers, the women were divided into the four job strain categories: high strain, active, passive and low strain. Gestational age at birth, birthweight and congenital malformations were extracted from the Danish Medical Birth Register. The outcome variable on asthma and atopic dermatitis were based on maternal self-reports from the fourth (child 18 months) and fifth (child seven years old) interviews/questionnaires. All studies in the thesis were based on protocols describing methods, analyses etc. prior to handling. No associations were found between exposure to high strain (high demands, low control) during pregnancy and preterm birth, small for gestational age, congenital malformations and asthma in the children when compared to women exposed to low strain (low demands, high control). A protective effect on large for gestational age was observed when exposed to high strain, suggesting an impact on the birthweight although it was not seen among the small for gestational age children. An association between high strain exposure and ever atopic dermatitis in seven-year-old children was observed, with 15% higher odds of atopic dermatitis when compared to women exposed to low strain during their pregnancy. The effects of high strain found were modest, but even modest effects might be relevant with high prevalence as with atopic dermatitis (lifetime prevalence of 15-20%). The studies included in this thesis add to the rather limited knowledge of prenatal exposure to work-related stress end health implications for the child. In the Danish work environment recommendations for women being pregnant or breastfeeding work-related stress is only mentioned very briefly. Findings from this thesis do not support rewriting in regards to risk of preterm birth, low birth weight or malformations. Results did show effects in regards to allergic diseases, but as the study is the only one of its kind, other studies are needed to replicate the findings if to reconsider the recommendations for pregnant women. Future research could benefit of validation of the use of two single-item measures of job strain compared to fully validated multi-item instruments. A shift toward more "functional endpoints" as overweight and cognitive behavioural disorders in the children could be of relevance.