Routine ultrasound is the method of choice for dating pregnancy

Dating by ultrasound in the first half of pregnancy has become a routine part of antenatal care in many units around the world. A recent commentary in this journal' questioned the accuracy of ultrasound and argued for the use of menstrual history with some modifications of Nagele's rule. This would be a regrettable and retrograde step. Recent research has shown conclusively that, where equipment and trained personnel are available, ultrasound is the method of choice for dating pregnancies. We present a review of these findings and put their significance into a clinical context. In comparing the measurement of gestational age by menstrual dates and by ultrasound, one has to look at systematic and random error. In each case, ultrasound performs considerably better. Systematic error states how accurately, on average, the measurement reflects the truth in the sampled population; this is important for definitions and policies based upon them, such as the timing of induction of labour for post-term pregnancy. Nagele's rule uses the mean length and predicts this to be 280 days2. Over 160 years later it is surprising how accurately this figure still reflects the mean length of pregnancy, including spontaneous and elective deliveries, in a database of 25,000 pregnancies with 'certain' menstrual dates: 279.7 days3. But in view of the negative skewness of gestational age distributions, and the possible influence of induction policies which may vary in different populations, the mean or even the median is not the best parameter to derive the 'typical' length of pregnancy. Instead, in large enough databases this can be derived from a modal value, found to be 283 days3. This is the same as that in the study by Bergsjo et al.4 and which Olsen and Clausen' now propose as the 'true' length of gestation. In contrast, using routine second trimester ultrasound the modal length of gestation was found to be 281 days in two recent large studies from different population^^*^ using different dating f~ rmulae~ ,~ . The accuracy of these formulae were tested in studies of assisted conception pregnancies, and their systematic error was in each case found to be less than a day8v9. A study looking at the basal body temperature shift in 1408 conception cycles'O also concluded that the average gestation of term pregnancies was 281 days. Therefore, the typical length of gestation is not 283 days. Olsen and Clausen suggest that the number of post term pregnancies could be reduced by using 283 days, but conclusions about the length of pregnancy cannot be derived from menstrual data as the lengths of the follicular phase before conception are not known. Random error assesses how consistently the measure is performing within the population. The problem with reliance on menstrual history for dating pregnancies is well known. Firstly, many women do not remember or are uncertain of the date of their last period' 'J2. Secondly, even if the menstrual dates are considered 'certain' or 'reliable', they have a much wider distribution which tends to be skewed towards an over-estimation of gestational age when compared with ~ l t r a s o u n d ~ J ~ ~ ' ~ * ~ . This variation is again related to the fact that in many instances conception occurs not in mid-cycle but later10J8J9. In a study of basal body temperature'O, the day of the temperature shift within the cycle had a wide distribution and positive skew, on average 16.5 days after the first day of the last menstrual period; whereas the interval between temperature shift and birth had a narrow and near-normal distribution, averaging 264.8 days. In a biochemical study of 75 ovulatory cycles, the average LH surge was also noted to be 16.4 days into the cycle19. Significantly, this study also showed that in half of the cycles in which ovulation was delayed until after day 18, this delay would not have been predicted on the basis of the length of the menstrual cycle. In contrast ultrasound error is normally distributed and has a much narrower error margin. Studies in assisted conception pregnancies showed that in single units, the random error with a formula for