The clinical significance of this finding is underlined by the association between fetal growth in early pregnancy and the risk of delivering a small for gestational age infant. Before the widespread use of ultrasound, fetal growth was thought to be largely genetically determined in early pregnancy and variation in fetal growth was thought to be primarily a feature of the second half of pregnancy.13 With the widespread clinical use of ultrasound, analysis of routinely collected data showed that fetuses that were smaller than expected in early pregnancy were at increased risk of adverse outcome, particularly poor growth. Intrauterine growth restriction was postulated to be evident in very early pregnancy.6 However, other authors suggested that a prolonged interval between menstruation and conception was the determining factor.7 Here we have shown that fetal growth in the first trimester of pregnancy is a major determinant of birth weight. Our study in pregnancies resulting from assisted reproductive technology has shown that the association between size in the first trimester and birth weight is not related to delayed ovulation but to first trimester growth.