An evaluation of the maternal effect on spontaneous abortion and the genetic contribution to various reproductive variables was made using like-sexed twins born between 1915–1946. Health and reproductive questionnaires were sent to 2,365 twin pairs listed in the Norwegian Twin Registry. Zygosity was determined by a questionnaire which proved to be 98% accurate in discriminating between monozygotic (MZ) and dizygotic (DZ) twins. Verification of the zygosity questionnaire was accomplished by blood cell marker zygosity determination in 200 twin pairs. Data from 428 monozygotic twin pairs (174 male pairs and 254 female pairs) were analyzed by a new model for qualitative traits and a striking maternal effect was found. The results suggested that over 54% of all women may belong to a high risk group who have approximately a 13% risk of early fetal loss. The incidence of first trimester abortions and spontaneous abortion reported by a sample of 915 like-sexed twin pairs and spouses was .079 and .089, respectively; there were no significant differences between male and female pairs.
Other reproductive variables were studied in both MZ and DZ twins including the age of menarche and menopause, the age at marriage and birth of the first child, and the interval between marriage and the first birth. These variables were examined for evidence of genetic effects and secular trends. The mean age of menarche had decreased significantly between the years 1915–1935 and 1936–1946. The secular trend appeared to be environmental in nature and accounted for more than 7% of the overall variation in age of menarche. Sixty-five percent of the variation in the age of menarche was attributed to genetic factors. The mean age of marriage and the mean age at the first birth decreased significantly between 1915–1935 and 1936–1946. Twins born between 1936–1946 also reported fewer pregnancies than did twins born between 1915–1946, but this was statistically significant among MZ twins only; no significant difference in the frequency of spontaneous abortions was found between the 1915–1935 and the 1936–1946 groups.
Although there were occasional groups in which differences were statistically significant, no general trends were identified in comparing couples who reported a spontaneous abortion with those who did not. Traits compared were: the age of menarche, age of marriage, age at first birth, the interval between marriage and first birth and the mean present age of the twins and spouses. Couples reporting a first trimester spontaneous abortion had significantly more total pregnancies than those not reporting a spontaneous abortion. Unaffected pregnancies were also significantly more numerous in couples with a spontaneous abortion compared to those without, suggesting over-compensation for fetal loss.
A maternal effect in spontaneous abortion was indicated by the correlation coefficients found; significant positive correlations were found in both MZ and DZ females but not in males. For total pregnancies and unaffected pregnancies, MZ twins, both male and female, had significant correlation coefficients while DZ twins did not.