A theoretical function is proposed which describes the individual weight-age relationship for foetuses in pigs. The function is based on total growth of the litter. A maximum uterus capacity, dependent on insemination weight of the sow, and a limitation of this capacity, dependent on litter size, is assumed. A modified Michaelis-Menten function is used as a basic growth function. The model was tested against experimental data and compared with four models from the literature. Goodness of fit, extrapolation outside the range of data, and biological interpretation of the parameters from the model were checked. A maximum uterus capacity of 20·3 kg for the average insemination weight of 119·0 kg was found. The inflexion point of the model was significantly (P < 0·05) dependent on litter size.