Objective: To investigate the fetal and maternal outcomes, risk factors of disease progression and adverse pregnancy outcomes (APOs) in patients with undifferentiated connective tissue disease (UCTD). Methods: This retrospective study described the outcomes of 106 pregnancies in patients with UCTD. The patients were divided into APOs group (n=53) and non-APOs group (n=53). The APOs were defined as miscarriage, premature birth, pre-eclampsia, premature rupture of membranes (PROM), intrauterine growth restriction (IUGR), postpartum hemorrhage (PPH), and stillbirth, small for gestational age infant (SGA), low birth weight infant (LBW) and birth defects. The differences in clinical manifestations, laboratory data and pregnancy outcomes between the two groups were compared. Logistic regression analysis was performed to analyze the risk factors for APOs and the progression of UCTD to definitive CTD. Results: There were 99 (93.39%) live births, 4 (3.77%) stillbirths and 3 (2.83%) miscarriage, 20 (18.86%) preterm delivery, 6 (5.66%) SGA, 17 (16.03%) LBW, 11 (10.37%) pre-eclampsia, 7 (6.60%) cases IUGR, 19 (17.92%) cases PROM, 10 (9.43%) cases PPH. Compared with the patients without APOs, the patients with APOs had a higher positive rate of anti-SSA antibodies (73.58% vs. 54.71%, P=0.036), higher rate of leukopenia (15.09% vs. 3.77%, P=0.046), lower haemoglobin level [109.00 (99.50, 118.00) g/L vs. 124.00 (111.50, 132.00) g/L, P < 0.001].Multivariate Logistic regression analysis showed that leucopenia (OR=0.82, 95%CI: 0.688-0.994) was an independent risk factors for APOs in UCTD (P=0.042). Within a mean follow-up time of 5.00 (3.00, 7.00) years, the rate of disease progression to a definite CTD was 14.15%, including 8 (7.54%) Sjögren's syndrome, 4 (3.77%) systemic lupus erythematosus (SLE), 4 (3.77%) rheumatoid arthritis and 1 (0.94%) mixed connective tissue disease. Multivariate Cox proportional risk regression analysis showed that Raynaud phenomenon (HR=40.157, 95%CI: 3.172-508.326) was an independent risk factor for progression to SLE. Conclusion: Leukopenia is an independent risk factor for the development of APOs in patients with UCTD. Raynaud's phenmon is a risk factor for the progression of SLE. Tight disease monitoring and regular follow-up are the key measures to prevent adverse pregnancy outcomes and predict disease progression in UCTD patients with pregnancy.