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Ultrasound Obstet Gynecol. 2006 Mar;27(3):311-5.

The practical application of a mathematical model to predict the outcome of pregnancies of unknown location.

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Early Pregnancy, Gynaecological Ultrasound and MAS Unit, St George's Hospital, London, UK.



A logistic regression model has been developed previously to predict which pregnancies of unknown location (PULs) become ectopics. This model was based on the human chorionic gonadotropin (hCG) ratio (hCG 48 h/hCG 0 h). The aim of this study was to evaluate the model in an early pregnancy clinical setting.


This was a prospective non-interventional observational study. Women classified as having a PUL had serum hCG and progesterone levels taken at presentation and 48 h later. At 48 h, nurse practitioners in the early pregnancy unit gave their subjective assessment of the likely pregnancy outcome based upon existing strategies to predict the outcome of PULs and their own previous experience. This was used as the basis for the clinical management of the women. The hormonal data were then entered into the model, which was available on the clinic computer in the form of a Microsoft Excel package, and the predicted outcome according to this model was recorded for each case. The model was weighted in favor of detecting an ectopic pregnancy at the expense of failing PULs and intrauterine pregnancies. All women were followed up until the final true diagnosis was established: a failing PUL, an intrauterine pregnancy or an ectopic pregnancy. The predicted outcome according to the model was compared with that of the subjective assessment of the expert operator and the true outcome after follow-up.


Data were obtained from 357 women classified as having PULs. The final clinical outcomes were: 162 (45.4%) failing PULs, 166 (46.5%) intrauterine pregnancies and 29 (8.1%) ectopic pregnancies. Subjective assessment utilizing current strategies gave sensitivities of 79.3%, 92.8% and 87.0% for the detection of ectopic pregnancy, intrauterine pregnancy and failing PUL, respectively. The model detected ectopic pregnancy, intrauterine pregnancy and failing PUL with sensitivities of 82.8%, 86.8% and 73.5%, respectively. There were no adverse outcomes.


This logistic regression model, based on the hCG ratio, can be used to predict the outcome of PULs, especially the ectopic pregnancies, with a high degree of certainty. It compares favorably with subjective assessment by experienced operators using current strategies to predict the outcome of PULs. The model can be used by those with limited knowledge or understanding of the behavior of serum biochemistry in the first trimester and in turn aid in the classification of PULs into those at low and those at high risk of ectopic pregnancy at 48 h.

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