The present invention provides a method for measuring the body fat percent and fat accretion during pregnancy, which preferably utilizes a Fourier transform near infrared spectroscopy (FT-NIR) fat determination method. The FT-NIR results can be used to measure and monitor body fat percent, and used to measure the mothers fat accretion rate. A method to monitor weight gain in a pregnant subject is provided wherein the body fat percentage of the subject is maintained at a constant level during gestation, and the subjects fat accretion rate is monitored so as to follow the fat accretion rate provided by using pre-defined fat accretion determination formulae. By measurement of the body fat percent and fat accretion rates, the weight gain of a subject can be monitored and/or controlled during pregnancy.
