Having discussed the various methods of dating pregnancy and their shortcomings, we will now discuss the best approach to ensure accurate dating of pregnancy.



1.  Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 weeks and 6 days of gestation) remains the most accurate method of establishing and confirming the age of pregnancy in pregnancies conceived by natural methods. This is especially true of the measurement obtained from the CRL (Crown rump length) and not the gestational sac.

2.  For pregnancy resulting from ovulation induction and intrauterine insemination of semen or ovulation induction and timed intercourse the pregnancy age should be calculated from the ovulaton date and for those conceived via in-vitro fertilization (IVF) and embryo transfer (ET), the pregnancy age should be calculated from the embryo transfer date, as this is the most accurate way of dating such pregnancies. For more information on how to do this please see DATING OF PREGNANCY FROM THE OVULATION DATE

3.  All women should endeavour to present in the Antenatal clinic within two months of missing their period. At booking, dating of the pregnancy must be done immediately from either the LMP (Last menstrual period), the ovulation date or the embryo transfer date for pregnancies conceived by IVF and compared with that from an early scan.

4.  If the difference in the date gotten from the LMP is more than 5 days from the date gotten from an ultrasound scan done between 7 weeks and 13 weeks and 6 days of pregnancy, then the ultrasound scan date should be picked over the date from LMP. Similarly, if the difference in the date gotten from the LMP is greater than 7 days from that gotten from a scan done between 14 weeks and 15 weeks and 6 days or greater than 10 days for a scan done between 16 weeks and 22 weeks,  then the scan dates must be picked over that calculated from the LMP. The date calculated from the ovulation date, and the embryo transfer date in pregnancies achieved by ovulation tracking with intrauterine insemination (IUI) and IVF respectively are always accurate and compatible with that from an early scan unless there is an error with the ultrasound scan or an error with  the documentation of the ovulation date/embryo transfer date.

5.  Once a pregnancy is dated from the LMP and confirmed by an early ultrasound scan, the pregnancy age and the “due date”/EDD should be well documented by the doctor in the woman’s note and the woman should be well notified of her EDD. The woman should be informed that this date is not alterable unless both the doctor and the patient agree that there was a major error in the initial calculation of her dates both from her LMP and the ultrasound scan. Any naturally conceived pregnancy without an ultrasound examination to confirm or review both the pregnancy date and the EDD before 22 weeks of age should be regarded as poorly dated.

6.  The earliest scan is always the most accurate scan and should be used in dating a pregnancy. The woman should be dully informed that any late pregnancy scan can not have any effect on the dating of pregnancy. A late scan is any scan done after the 22nd week of pregnancy and they should not be used in dating pregnancy if an earlier scan has been done.

7.  In a situation where a woman presents after 20 weeks of pregnancy and she can vividly remember her LMP, by using the 8% error margin of ultrasound dating, a scan should be done as early as possible and the error margin calculated. If the EDD calculated from the LMP is within the error margin of the ultrasound scan, i.e. the difference between the “due date” calcuated from the ultrasound date and the due date calculated from the LMP is less than the error margin of the ultrasound scan, then the EDD calculated from the LMP should be used, but not so if the date is outside the error margin, where in the ultrasound scan date should be used instead. Because the error margin can be up to 21 days for ultrasound scan dating of pregnancy in the 3rd trimester, when using the scan in the third trimester to calculate the due date, in cases where the due date from the LMP is outside the error margin of a late scan for a woman whose pregnancy has not been previously dated, I personally counsel that such women be delivered by the 40th week of pregnancy to avoid the risk of prolonged pregnancy at the same time prevent a preterm delivery of the baby. Similarly, for women who are unsure of their LMP but present in the third trimester, using a similar approach to the above, the ultrasound scan must be done at presentation and the due date used to plan for her delivery, granted the baby’s clinical parameters remain healthy. A brief  illustration of this is given below.

For example, a woman who presents for booking at 29 weeks of pregnancy a LMP of 15/01/18. She was yet to do any ultrasound scan in the pregnancy. An ultrasound scan was done and the EDD as calculated by the ultrasound scan is 11/10/18 +/- 2 weeks. Because the EDD from her LMP is 15+7/1+9/18 = 22/10/18, and the difference between the EDD from her LMP and that from her scan is 22-11=11 days. Her EDD from her LMP will be used for dating since the difference between the two EDD is less than the error margin of 2 weeks or 14 days stated on the scan. But if the EDD was for example 02/10/18, which is about 20 days different from the EDD calculated by the LMP, then the EDD from the scan will be used to date the pregnancy and plan the delivery since 20 days is more than the error margin of 2 weeks stated by the ultrasound scan. However, if an early scan was done before 22 weeks of pregnancy and it was compatible with the EDD calculated from the LMP, then any other ultrasound scan done after will be irrelevant in dating the pregnancy as mentioned earlier. For more information of this and clarification about your pregnancy date, please CHAT WITH OUR CONSULTANTS.