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Data from the Medical Birth Registry of Norway, covering more than one million births for the period 1967-1984, were used to study the magnitude and effects of selective fertility, which is the tendency for a woman to replace a perinatal loss. Variation in fertility after the first three births is studied, controlling for perinatal outcome of previous births, maternal age, and year of birth. Even after the first birth, fertility is higher after a perinatal loss. Selective fertility is more strongly present at each successive birth order, and at each birth order it is stronger among older women. As the average number of births per woman decreases, the force of selective fertility increases; that is, its importance has increased over time. Perinatal mortality at the third and fourth birth orders is particularly distorted by the mechanism of selective fertility in studies based on cross-sectional data. Mortality at second birth is exaggerated by 1%, at third birth by 8% to 20%, and at fourth birth by 18% to 27%, with the largest effects seen in the later periods. A major portion of the increase in perinatal mortality from the second to fourth birth seen in most studies based on cross-sectional data can be explained by the mechanism of selective fertility.

The effect of marital status on perinatal mortality is studied for births in Norway, 1967-1981, in terms of relative risk (RR) of unmarried vs. married mothers. The births were linked in units of sibships. At first birth, RR decreased from 1.86 in 1967-1971 to 1.32 in 1977-1981. At second birth the perinatal mortality varied greatly according to marital history with a RR of 2.03 when comparing sibships where mothers were unmarried at both pregnancies with mothers married at both pregnancies. Almost no excess risk was demonstrated for mothers that were married at second birth, but unmarried at first. These mothers appeared to have no excess risk even at the first birth in spite of being unmarried. The results suggest that marital status is still a risk factor to be considered, even in the Scandinavian countries where the marginal effect of marital status in recent years has been reported to be low. To an increasing extent, the high risk women, previously identified by marital status, are now hidden in a larger low risk group which apparently has more stable family relations, e.g. tending to be married at second birth.

Knowledge of the likelihood of a repetition of sudden infant death syndrome within a sibship, particularly in the next sibling, is important to parents. Methodologic considerations with respect to the studies of recurrence risk already published indicate that the rates reported are overestimates. This suspicion is confirmed by our study based on 826,162 infants surviving the first week of life on file in the Medical Birth Registry of Norway. A total of 1062 (1.3 per 1,000) infants died of SIDS; five deaths occurred as the second case in a family. The recurrence risk for the nextborn sibling was 5.6 per 1000, and for all subsequent siblings 4.8 per 1000, which would seem encouragingly low from a counseling point of view.

Epidemiologic surveillance in Norway, the United States, Nigeria, Japan, Venezuela, India, and China, covering periods from 1851 to 1981, demonstrates a consistent decline in incidence rates of leprosy. At the same time, secular trends have been observed which imply an increasing age at onset, an increasing male excess, and an increasing fraction of new cases represented by multibacillary leprosy. Theoretically, an increasing age at onset may be caused by two mechanisms, namely postponement of infection to a later age and/or an increasing fraction of patients with long incubation periods. Cohort analyses have shown no increase in age at onset in subsequent birth cohorts, but rather have shown a decrease. The latter mechanism, the increasing importance of long incubation periods, is consistent with the shift toward multibacillary cases in which the incubation period is longer than that in paucibacillary cases. Apparently, this mechanism has also been present during the decline of tuberculosis. An increasing fraction of new patients with long incubation periods, resulting in an increasing age at onset, is proposed as a general principle to be expected in any disease in rapid decline which also has a long and varying incubation period. This theory offers a basis for assessment of secular trends.

The considerable variation in leprosy prevalence among 35 villages and field areas in South India was compared with variations in their socioeconomic and nutritional conditions. Using villages/field areas and aggregates thereof as observational units, leprosy prevalence rates were found to be significantly lower in field areas than in villages. Moreover, there was a significant correlation between the occurrence of malnutrition in children 1 to 4 years of age and the prevalence of leprosy. The possible association between the occurrence of malnutrition in childhood and leprosy is discussed in light of the present knowledge of leprosy, immunocompetence, and malnutrition. Strategies for further illumination of the hypothesized importance of malnutrition among children are discussed.