Table 4.

Linear regression results for the effects of bearing twins on average inter-birth intervals. (AFB, age at first birth; ALB, age at last birth; LDS, Latter-day Saints.)

dependent variable: average inter-birth interval
p > F ≤ 0.0001, r2 = 0.1086p > F ≤ 0.0001, r2 = 0.0897
mother's birth year before 1870, n = 20 350mother's birth year after 1870–1900, n = 31 909
95% confidence limits95% confidence limits
variableβlower CIupper CIp>χ2βlower CIupper CIp>χ2
intercept−8.1086−9.4646−6.7526<0.0001−18.4527−20.5780−16.3275<0.0001
ever mother of twins−0.0219−0.04970.00590.1228−0.0571−0.0918−0.02240.0013
birth year0.00560.00490.0063<0.0001−0.0688−0.0716−0.0660<0.0001
AFB−0.0100−0.0123−0.0078<0.0001−0.0162−0.0188−0.0137<0.0001
ALB0.01490.01300.0167<0.00010.02780.02600.0296<0.0001
children died before 18−0.1242−0.1299−0.1185<0.0001−0.2153−0.2244−0.2062<0.0001
husband lifespan0.00180.00110.0025<0.00010.00250.00180.0032<0.0001
LDS religion−0.0965−0.1144−0.0787<0.0001−0.0332−0.0525−0.01390.0008