Kallmann syndrome (KS) patients carrying FGFR1 mutations can transmit the disorder to their offspring as can asymptomatic female carriers of mutations in KAL1. We describe for the first time two cases in which KS was suspected during fetal life because of the family context and malformation detection by fetal ultrasound: syndactyly or unilateral renal agenesis in subjects with respectively FGFR1 and KAL1 mutations. In relevant family history, ultrasound monitoring can detect KS associated signs before birth and thus enable neonatal diagnosis and early management. These observations also underline the importance of genetic counselling for patients who may transmit KS to their offspring.