Descriptive statistics of the female TwinsUK and Lifelines samples.

<p>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.</p><p>Descriptive statistics of the female TwinsUK and Lifelines samples.