Volum e 166 Number 5 12. Manning FA, Platt I.D, Sipos L. Antepartum fetal evalu- ation: development of a fetal bioph ysical profile scor e. AM J OBST ET GYNE COL 1980; 136 :787-95. 13. Pearce JMF, Campbell S, Coh en-Overbeck T, Hackett G, Hern an adezJ , RoystonJP. Reference ranges and sources of variation for indices of pulsed Doppler flow velocity waveforms from the uteroplacental and fetal circulation. Br J Obstet Gynae col 1988;95:248-56. 14. Deter RI., Harrist RB, Hadlock FP, Carpenter RJ. Fetal head and abdominal circu mfere nces. II. A crit ical re-eval- uation of the relationship to mens tru al age. JCU 1982 ; 10:365-72. Fetal assessment by three ultrasonographic characteristics 15. Vintzileos AM, Campbell WA, Ingardia CJ . Fetal bio- ph ysical profile and its pr ed ictive value. Obstet Gynecol 1983;62:271-8. 16. Pillai M, James D. Hiccups and breathing in human fe- tuses. Arch Dis Child 1990;65: 1072-5. 17. McNay MB. Diagnostic ultrasonography. Clin Obstet Gy- necol 1987; I :29·53. 18. Roberton NRC. The care of ne onates with respiratory failure. Recent Adv Perinat Med 1983;1:169-90. 19. Ten Centre Study Group. Ten centre trial of artificial sur factant (artificial long expanding compound ) in very pr emature babies . BMJ 1987 ;294 :991-6 . A model-based prediction for transvaginal ultrasonographic identification of early intrauterine pregnancy Bruce S. Shapiro, MD: Michael Escobar, PhD: Robert Makuch, PhD,b Gad Lavy, MD: and Alan H. DeCherney, MD- New Haven, Connecticut OBJECTIVE: Our objective was to develop a mathematic model for the prediction of transvaginal ultrasonographic identification of intrauterine gestation as a function of human chorionic gonadotropin titer or gestational age. STUDY DESIGN: In this prospective, descriptive study normal intrauterine pregnancies of 31 patients from the Yale in vitro fertilization and embryo transfer program and infertility clinic were studied. Logistic regression analysis was used to develop a probabili1ycurve of transvaginal ultrasonographic detection of intrauterine pregnancy as a function of human chorionic gonadotropin titer or gestational age with a 5 MHz vaginal transducer. RESULTS: A model-based prediction was constructed. It revealed that nearly all intrauterine sacs should be identified by a human chorionic gonadotropin titer of 3000 mlU /ml (first international reference preparation) or by a gestational age of 37 days on the basis of the 5 MHz vaginal transducer. CONCLUSIONS: A model-based prediction for transvaginal ultrasonographic identification of early intrauterine pregnancy is presented that can be readily adapted by individual institutions using their own ultrasonographic equipment and human chorionic gonadotropin assay . (AM J Ossrsr GVNECOL 1992;166:1495-500.) Key words: Trans vagin al ultrason ograph y, mathem atic model, early pregnancy T he wide spread availability of a radioimmunoassay for human chorionic gonad otropin furnished the cli- nician with the ability to make an early diagnosis of pregnancy. The development of high-resolution ab- From the Department of Obstetrics and Gynecology" and the Division of Biostatistics: Yale University School of Medicine. Received for publication August 19, 1991; accepted October 31 , 1991. Reprint requests: Bruce Shapiro, MD , Department of Obstetrics and Gynecology, University of Nevada School of Medi cine, 2570 S. Bruce sc, Suite D-7, Las Vegas, N V 89 109. 611 134748 dominal ultrasonograph y provided the clinician with the opportunit y to make the early diagnosis of intra- uterine gestation. The maj or clinical importance of this adva nce was in the earlyexclusion of ectopic pregnancy. The confirmation of an intrauterine pregnancy is con- sidered sufficient to exclude ectopic gestation because hetertot opi c pregnancy is relatively uncommon.':' The absence of an identifiable int rauterine gesta- tional sac on abdominal and transvaginal ultrasono- graphic examination when the human chorionic go- nad otropin titer has reached a defined threshold level has been used widely as a criteria for surgical explo- 1495