OBJECTIVES Reference or standard curves are required in many biomedical problems. Values which lie outside the limits of these reference curves may indicated the presence of disorder. Data are from the French EDEN mother-child cohort [1]. We are studying fetal weight (in grammes) that depends on the gestional age (GA) in the second and the third trimesters of mother's pregnancy. Some classical parametric and semi-parametric methods as polynomial [2][3] and LMS method [4][5] or nonparametric methods as kernel estimation [6] are used to construct these curves. However, some of them requires strong assumptions.