Abstract
The accuracy and precision of gestational dating, based on single measurements of the fetal biparietal diameter (BPD), was estimated in a consecutive series of 970 apparently normal, singleton pregnancies. The BPD sizes used varied between 11 and 60 mm, corresponding to 9–22 postconceptional weeks. The ‘conceptual ages’, used for reference, were estimated by means of ultrasonic measurements of the fetal crown—rump lengths (CRL). The association between the estimated conceptual age (dependent variable) and BPD size (independent variable) was found to be well represented (R2 = 0.972) by a second order polynomial: conceptual age = 44.7 + 1.069 × BPD+ 0.01382 × BPD × BPD. To check the validity of the proposed equation, we used 23 fetuses conceived by artificial insemination with donor semen as controls. There was good agreement between the virtual conceptual ages of the control cases and their corresponding BPD-estimated conceptual ages (0.5 days mean difference, 3.6 days SD) when the suggested regression equation was used. The estimated precision of BPD-dating was minumum ±4.4 days (= ±2SD) at 9–10 completed postconceptional weeks. At 14 completed weeks the corresponding precision was found to be ±11 days (±2SD). At the end of the studied BPD size interval, i.e. 56–60 mm BPD corresponding to 22 conceptual weeks, the precision was ±15 days. From an obstetrical point of view it seems obvious that the imprecision of gestational dating associated with BPD measurements > 35 mm should call for ultrasonic dating procedures to be performed earlier in pregnancy. An optimal precision of dating, i.e. better than ±6 days (= ±2SD), will certainly be achieved by ultrasonic measurements of the BPD between the 9th and 12th completed postconceptional weeks.
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