The authors attempted the ultrasound determination of fetal sex in the first trimester of pregnancy. The study included 240 pregnant women between 5-13 weeks of pregnancy. The crown-rump length (CRL) of male and female fetuses was compared. Statistical analysis of the results showed a significantly faster increase in CRL in male fetuses starting from the 7th week of pregnancy. This effect can be linked to the effects of fetal testosterone. The obtained results indicate the possibility of the forecast, with high probability, the sex of the fetus from the 7th week of pregnancy.