Fig. 2
Equation for the calculation of pre-pregnancy body mass index-specific gestational weight gain z scores based on a Box-Cox t modela. awhere Y is weight gain at a certain gestational age, L is lambda, M is mu, and S is sigma. The random variable Z is assumed to follow a t distribution with degrees of freedom, Tau > 0, treated as a continuous parameter. The parameters of our Box-Cox t model for each pre-pregnancy body mass index group are provided for the rounded gestational ages. This equation can be applied on data using the y2z function of the AGD package in R. The function will allow the calculation of z scores for the exact gestational age by extrapolating the parameters. For applying the equation or function, weight gain must be > 0, because the model cannot deal with negative values. In order to fit the Box-Cox t model, parameters were calculated based on weight gain + 20 kg, and thus 20 kg must be added to weight gain to be able to use our parameters. The constant of 20 kg was chosen since − 20 kg is an extremely low value for weight change during pregnancy. After adding the 20 kg, weight gain must be > 0; otherwise, the equation or function using our Box-Cox t model parameters cannot be applied for the remaining ≤ 0 values