Fetal intra-uterine growth charts are generally based on last menstrual period (LMP) data. Such charts may not really be applicable to infants whose gestational age is ultrasonically determined in early pregnancy. 1369 consecutive singleton pregnancies were studied. All fetuses had their gestational age estimated both by ultrasonic measurement of fetal crown–rump length and from registered last menstrual period data. All infants were weighed at birth as a part of the routine care. The age determination according to the two methods differed in 664 out of 1369 cases and in 516 of these 664 cases the age according to LMP was higher than according to CRL – a highly significant difference (p < 0.001). Mean values for birth weight were found to be larger when correlated to age in terms of CRL determinations than when related to age as calculated from LMP data. Birth weight also showed a significant increase between 40 and 42 weeks when age was determined ultrasonically. This was not the case when age determination was based on LMP data. The explanation of this fact is that age is overestimated in late pregnancy when calculated from LMP data. These facts ought to be considered when future fetal growth charts are constructed.