Descriptive statistics of the female TwinsUK and Lifelines samples.

Note that the N for the age at first birth (AFB) is different from the N for number of children ever born (NEB). The reason for this is that only women older than 45 have been included in the analysis of NEB. For example, a 35 years old woman with a first child is part of the analysis for AFB but not for NEB. Therefore in the Lifelines cohorts the N for AFB is larger than for NEB, because it contains a large proportion of women younger than 45. This also implies that the average AFB in the more recent birth cohorts is younger than in the overall population.
Descriptive statistics of the female TwinsUK and Lifelines samples.