This study aims to investigate the effects of progesterone on the possible changes in nuchal translucency (NT) levels for patients diagnosed with threatened miscarriage. The study group was composed of 125 patients diagnosed with threatened miscarriage and taking 400 mg/day micronized orally progesterone at least for two weeks, the control group was composed of 160 healthy pregnant women not taking any progesterone. Crown rump length (CRL) NT thickness, Pregnancy-associated plasma protein-A (PAPP-A), free beta human chorionic gonadotropin (Beta-HCG) levels of patients were measured for assessment of aneuploidy risk. Both of the groups were divided into four subgroups to determine the relationship between thickness of NT and progesterone use for specific CRL measurements. CRL in the first, second, third and fourth group was 45-55 mm, 55-65 mm, 65-75 mm, 75-84 mm, respectively. The two groups were age and BMI matched. In all groups of CRL there were no significant difference in Mom levels of NT thickness, PAPP-A and free Beta-HCG between the study and control groups. There havent been any relation between NT thickness and progesterone use.IMPACT STATEMENT What is already known about this subject?Recently some studies have claimed that progesterone use might have caused atypical blood flow pattern on foetal circulation, which could possibly increase NT. If the NT thickness is affected by the use of progesterone, then the false positive rate of detecting Down Syndrome screening tests would increase. What the results of this study add?In this study we did not found any relation between NT thickness and progesterone use. What the implications are of these findings for clinical practice and/or further research?Using orally progesterone due to threatened miscarriage do not change NT thickness levels. Further studies have to be done with a large number of participants.