Timing of TAPS Diagnosis
Twin anemia polycythemia sequence (TAPS) is typically diagnosed in the late second or third trimester, with a median gestational age of approximately 23-24 weeks, though it can present at any time from 16 weeks through the end of the third trimester. 1, 2
Gestational Age at Diagnosis
TAPS can present at any time during the second or third trimesters, distinguishing it from twin-twin transfusion syndrome (TTTS), which typically presents earlier. 1
In a large international cohort of 249 cases, spontaneous TAPS was diagnosed antenatally at a median gestational age of 23.7 weeks (interquartile range 9.7-28.8 weeks; range 15.1-35.3 weeks). 2
TAPS occurs significantly later in pregnancy than TTTS, with median gestational age at diagnosis of 26.0 weeks for TAPS compared to 20.4 weeks for TTTS. 3
The condition most commonly manifests in the late second or third trimester, particularly after 20 weeks of gestation. 4
Surveillance Timing for Detection
Screening for TAPS should begin at 16 weeks of gestation using middle cerebral artery peak systolic velocity (MCA-PSV) Doppler measurements in all monochorionic twin pregnancies. 1
Surveillance should continue at least every 2 weeks throughout the second and third trimesters until delivery to detect TAPS at any stage. 1, 5
Once TAPS is diagnosed, monitoring should be scheduled weekly to assess disease progression and guide management decisions. 5
Clinical Context
Unlike TTTS, which has a more predictable gestational age window, TAPS has a broader and later presentation pattern, making consistent surveillance throughout pregnancy essential. 2, 3
Post-laser TTTS cases require particularly vigilant MCA-PSV surveillance as iatrogenic TAPS can develop after fetoscopic laser surgery, representing a substantial risk requiring another intervention. 1
The wide gestational age range at diagnosis (15-35 weeks) underscores the unpredictable nature of TAPS onset and the need for sustained monitoring protocols. 2
Important Caveats
TAPS is unlikely to be prenatally detected without systematic MCA-PSV evaluation, especially at earlier stages, making routine Doppler surveillance critical. 1
Approximately 12% of TAPS cases are diagnosed only postnatally, highlighting the challenge of antenatal detection even with surveillance protocols. 2
The later gestational age at diagnosis compared to TTTS does not indicate lower severity—overall mortality for TAPS is similar to TTTS despite its later onset. 3