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Screening for trisomy 21 in twin pregnancies in the first trimester: an update of the impact of chorionicity on maternal serum markers.
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Maternal serum free beta-hCG at 10 to 14 weeks of gestation in trisomic twin pregnancies.
Noble PL, Snijders RJ, Abraha HD, Sherwood RA, Nicolaides KH.
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