Since the advent of the ultrasound scan over 60 years ago, improvement in technology has rendered ultrasound scan invaluable in the field of Obstetrics and Gynaecology. Among its numerous indications, is its use in the first half of pregnancy as the most accurate means of dating pregnancy.



Just as babies born on the same day may not have much to set them apart in terms of length and weight, but grow up to have different heights and weights, due to differences in genetics, nutrition and other environmental influences, unborn babies at conception and in early pregnancy have similar features that can be measured to give accurate dating of pregnancy, which becomes less accurate as the pregnancy age increases due to the unborn babies’ genetic composition, the influence of the internal environment within the womb and associated maternal comorbidities. Hence, as the pregnancy age increases, the ultrasound scan becomes less accurate for dating pregnancy.

Similarly, clinical researches suggest that in expert hands, the pregnancy age and the due date when measured with a scan is associated with an 8% error margin expressed in days that is constant throughout pregnancy. This implies that the pregnancy age when measured early in pregnancy has a lower error margin expressed in days than when compared to that measured at an older age. For example, measuring the pregnancy at 9 week is associated with an error margin of 5 days (8% of 9 x 7days), while measuring the same pregnancy at 27 weeks is associated with an error margin of 15 days (8% of 27 X 7days); this suggests that the pregnancy date, when measured at 9 weeks may be wrong by 5 days but when measured at 27 weeks may be wrong by as much as 15 days or 2 weeks.

Therefore, from the above two illustrations, it is obvious that the most accurate time to date pregnancy with an ultrasound scan is sometime within the first three months of pregnancy i.e. the first trimester. But more specifically, the most accurate parameter for dating pregnancy is the measurement of the crown-rump length (CRL) of the fetus between the 8th and 11th week of pregnancy. This is because, the CRL, which is defined as the measurement of the length of human fetuses from the top of the head (crown) to the bottom of the buttocks (rump), shows little or no variation when measured in different embryos at the same age of pregnancy between 8 weeks and 11 weeks. Secondly, this stage of pregnancy corresponds to a period when there is a very rapid growth of the fetus which can then be used to accurately date the age of the fetus to the nearest days. Finally, at this period, the fetus is relatively fixed and non-mobile ensuring both easy and reproducible measurement of the CRL. The measurement of the gestational sac (GS) for dating pregnancy as early as 3 to 5 weeks of pregnancy to about 7 weeks of pregnancy is less accurate compared to the CRL. This is because the GS is merely an enclosing bag that contains the embryo and its surrounding fluid/water and not a direct appendage of the fetus. As pregnancy advances beyond the first trimester into the second trimester, various body parameters such as the bi-parietal diameter, head circumference, femur length, abdominal circumference can be used to measure the pregnancy age, albeit with more variation and thus with less accuracy compared to the measurement of the CRL at 7 to 13 weeks of pregnancy.

Knowing therefore that although the ultrasound scan dating is superior to that calculated by the LMP (Last menstrual period), and appreciating also that ultrasound scan dating of pregnancy becomes less accurate as the pregnancy age advances, the ultrasound scan date should only be picked over the date calculated by the LMP, if the difference between two dates is greater than 5 days for a scan done between 7 weeks to 13 weeks, or greater than 7 days for a scan done between 14 weeks to 16 weeks, or greater than 10 days for a scan done between 16 weeks and 22 weeks, irrespective of how sure the mother is of her LMP. However, any scan done after 22 weeks of pregnancy, should not be used in dating pregnancy except it is the last resort as the error margin in these instances can be up to 2 weeks or more. 



1.  The accuracy of the ultrasound scan depends greatly on the skill of the person performing the examination. This is especially important as wrong results may adversely mislead the total course of pregnancy or cause confusion in the management of a patient.

2.  The quality of the ultrasound scan image has a significant effect on the interpretation of the features seen on an ultrasound examination, thus going a long way to affect its accuracy.

3.  Other factors that may affect the accuracy of an ultrasound scan include the examined patient’s size and the position of the fetus at the time of the examination. The bigger a patient is, the poorer the quality of the image produced by the ultrasound scan.

4.  Similarly, as the pregnancy advances, due to increased activity and increased size of the fetus, the position of the fetus and the movement of one part of its body relative to the others may obscure the measurements of some fetal parameters necessary for dating. Luckily this can be overcome with patience.

5.  Finally in some parts of the sub-Sahara Africa and many developing parts of the World, not only are properly trained personnel short in supply, quality and good ultrasound scans are often difficult to come by, resulting in many women lacking the opportunity to have their pregnancy properly dated. Worse still, many of these women have a habit of presenting late in pregnancy for antenatal booking and many others are unsure of their LMP. For more information on ultrasound dating of pregnancy, please CHAT WITH OUR CONSULTANTS.