Anthropometry and puberty assessment

Trained members of the Jersey Girl Study staff recorded the height and weight at study baseline and calculated BMI (weight in kilograms divided by the square of height in metres). BMI-for-age percentiles and Z-scores were calculated using the SAS program based on the growth charts of the Centers for Disease Control and Prevention⁽,²⁹⁾.

Puberty was evaluated using the Tanner scale, which ranges from stage 1 (prepubertal) to stage 5 (post-pubertal)⁽,³⁰⁾. Tanner stage was reported by the girls’ mothers using a form with pictorial representations and descriptions of each stage; a physician also examined the girls and filled out the same form at the baseline study interview. Thelarche at baseline was defined as a Tanner stage of 2 or greater, as reported by the physician (or as reported by the mother in the sixteen girls who did not have physician staging). Our previous analysis of agreement between mother and physician Tanner staging for breast development found agreement to be high (over 85 % agreement, κ = 0·7)⁽,³¹⁾.

Age at thelarche was determined using two different questions. First, at baseline and on each follow-up form, mothers were asked if girls’ breasts had started to develop, and if so, at what age. Second, mothers were asked to provide an update on Tanner staging, as discussed above, at baseline and each annual follow-up (until menarche). Age at thelarche was based purely on mothers’ report, since physician validation was available only at the time of the baseline interview. If the mother reported breast development at baseline, and reported an age of breast development, this was used as age at thelarche. When this age was missing or there were inconsistencies in the mother’s report, we determined age at thelarche as follows. If the mother reported a Tanner stage of 2 or greater before the reported age at thelarche (or age at thelarche was missing; n 36), the age at report of Tanner stage 2 (or 6 months before, in cases of girls aged 10 years or older at report) was used as an estimate of the correct age at thelarche. If a mother estimated an age in years but not months (n 16), the age in years plus 6 months was used. If the mother’s estimate at the subsequent follow-up was similar but more precise (n 2), that estimate of age was used.

Date of the first menstrual period was asked during the baseline interview and on each subsequent annual follow-up questionnaire. If only the month and year were provided, the 15th of the month was imputed as the date of first menstrual period.