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Tunon, K., Eik-Nes, S. H., Grøttum, P., Von Düring, V., Kahn, J. A. (2000). Gestational age in pregnancies conceived after in vitro fertilization: A comparison between age assessed from oocyte retrieval, crown-rump length and biparietal diameter. Ultrasound in Obstetrics and Gynecology15 (1): 41–46. PMID10776011. DOI: 10.1046/j.1469-0705.2000.00004.x.