OBJECTIVE: Monochorionic diamniotic (MCDA) twins with amniotic fluid abnormalities that do not meet criteria for twin-twin transfusion syndrome (TTTS) concern physicians and families. This study aimed to describe the natural history of amniotic fluid abnormalities. METHODS: In this retrospective case-control study, TTTS screening ultrasounds and clinical records throughout all MCDA twin gestations were reviewed between 2018 and 2022 at a tertiary fetal care center. Twin pairs with normal imaging throughout gestation comprised a control group. Cases with fluid abnormalities were divided into four subgroups: (1) isolated polyhydramnios of one twin, (2) isolated oligohydramnios of one twin, (3) subjective physician concern about fluid discrepancy without objectively abnormal fluid in either twin, (4) or selective fetal growth restriction (sFGR) with any of the above fluid abnormalities. Since the aim was to trend unclear fluid findings, twins who flipped from normal imaging to clear diagnostic criteria for TTTS, selective fetal growth restriction (sFGR), or twin-anemia polycythemia sequence (TAPS) within one scan were excluded. Also excluded were gestations with structural anomalies affecting fluid quantity, higher order multiples, and cases with only one scan available. RESULTS: Of 737 gestations, 300 were excluded for clear diagnostic findings, 19 had a fetal anomaly impacting fluid, 10 were higher order multiples, and 44 had only one scan. Of the remaining 364, 224 had normal imaging and formed the control group, while 140 had fluid abnormalities not diagnostic for TTTS. Twins with fluid abnormalities developed more TTTS (41 %), TAPS (8 %), new-onset sFGR (11 %), and need for laser (36 % vs 0 %, all p <
0.001). Fetal demise (11 % vs 1 %, p <
0.001) and maternal admissions for fetal surveillance (15 % vs 4 % p = 0.001) were seen more often in cases, who also had an earlier gestational age at delivery (33.1 vs 35.1 weeks, p <
0.001) when compared to controls. CONCLUSIONS: Amniotic fluid abnormalities that do not yet meet criteria for TTTS should be considered a warning sign in MCDA twin pregnancies. These abnormalities are associated with more complications, fetal intervention, fetal demise, and long hospital stays.