Objective: To compare the predictive accuracy of the Gaussian and FMF algorithms for preeclampsia (PE) and small for gestational age fetuses (SGA). Design: Secondary analysis of a prospective cohort study. Setting: Tertiary referral hospital. Population: 2641 singleton pregnancies attending routine first-trimester scan from October 2015 to September 2017. Methods: Maternal characteristics, mean arterial blood pressure (MAP), and mean uterine artery pulsatility index (UtAPI) were recorded at the first-trimester scan. Serum placental growth factor (PlGF) and pregnancy-associated plasma protein-A (PAPP-A) were assessed between 8+0 and 13+6 weeks of gestation. Main outcome measures: The areas under the curve (AUC) for the predictive performance for early-onset (delivery <34 weeks) and preterm (delivery <37 weeks) PE, and early-onset (delivery <32 weeks) and preterm (delivery <37 weeks) SGA, were calculated with the Gaussian and FMF algorithms, and were subsequently compared. Results: Among the 2641 participants, 30 (1.14%) developed preterm PE, including 11 (0.42%) early-onset PE. Among the 2483 newborns, 44 (1.77%) were preterm SGA, including eight (0.32%) early-onset SGA. The FMF and the Gaussian algorithm showed a similar predictive performance for most outcomes and marker combinations. Conclusions: This study shows that the first-trimester Gaussian and FMF algorithms have similar performances for PE and SGA prediction. Accuracy of the FMF algorithm was similar to that reported in the original studies, adding evidence to its external validity. Funding: none Keywords: preeclampsia, screening, PlGF, early-onset preeclampsia, uterine artery Doppler, first trimester Tweetable abstract: The first-trimester Gaussian and FMF algorithms have similar predictive performances for preeclampsia and small-for-gestational-age fetuses.