To elucidate the cause of the wide variation of sensitivity of the nuchal-fold thickness (NFT) for a noninvasive prenatal screening test for fetal Down syndrome, we consecutively measured the NFT throughout pregnancy in eight fetuses with Down syndrome and 100 normal fetuses (negative controls) using video-recorded ultrasonography. When 6 mm was set as the cutoff value, 6/8 Down syndrome fetuses showed a NFT above this value at least once during pregnancy. However, the thickening was transient in 5 of them and resolved spontaneously during the second trimester. Persistent thickening of nuchal-fold was only observed in one fetus. From the results of the present study, we recommend that the NFT be measured repetitively during the first and second trimesters as a noninvasive prenatal screening test for Down syndrome.