Women’s LMP dates are just as accurate as scans…

small_8334277572Just a few weeks after I shared details of a research study which looked at pregnancy length, the International Journal of Gynaecology and Obstetrics has published the results of research which compared women’s estimated dates of birth as worked out from their LMP dates (eDOB) and ultrasound scans at a variety of gestational points (aDOB) with the date on which they actually gave birth.  (All of the women, of course, went into spontaneous labour.)  While the research is retrospective and thus only as good as the notes which professionals kept at the time, the study is very large and includes data from 18,708 women.

In Your Own Time was written to help parents and professionals better understand the issues and the evidence relating to the current induction epidemic. Looks at the evidence relating to due dates, ‘post-term’, older and larger women, suspected big babies, maternal race and more.

In short, the researchers found that there were no meaningful differences between the prediction of date of birth by scan and last menstrual period (LMP).  Which means, they conclude, that if the woman feels her own menstrual dates are reliable, there is no need for an early dating scan.  This is good news for a couple of reasons.  Firstly, because we really don’t know what the effects of such early scans are on developing babies, so it is good to know that these are unnecessary.  (The authors go on to suggest that the best time to perform a scan to date pregnancy if one is needed is 11-14 weeks, although the rationale for this isn’t well explained).

But of just as much concern for me is the move that has occured over the past few decades whereby technologically-derived knowledge has become seen as superior to women’s own knowledge, which also includes the knowledge that birth attendants can gain from women’s experience of being in their bodies.   And I don’t say that simply out of some concern for women’s self-esteem (although of course that’s important). I say it because I know that there are things we can know from women and their knowledge of their bodies that no machine will ever be able to tell us, and we need women to understand that this important kind of knowledge is valuable so that we don’t lose our connection with it.  This study further confirms the value of one element of women’s bodily knowledge and shows that it is not bettered by technology, and I hope that everyone reading this will find a way to tell at least one pregnant woman about it.

One particular finding from this study that may be of interest was that, although only 1 in 20 births occur on the eDOB, approximately 66% of births occur within a week (± 7 days) of the eDOB.  For those who choose to take the wider window* approach with women, this may be a useful number to share, although it may be important to be clear that this means that 66% of births occur within the fortnight that includes a week either side of the due date rather than within one week.

* If you don’t know what I mean by this, you might like to check out this post 😀

Sara Wickham’s bestselling book explains the process of induction of labour and shares information from research studies, debates and women’s, midwives’ and doctors’ experiences to help women and families become more informed and make the decision that is right for them.

If you’d like to know more, you might enjoy Inducing Labour: making informed decisions.




In a retrospective study, data were analyzed from 18 708 women with spontaneous labor who delivered a single neonate without major anomalies in a local health district in Australia between 2007 and 2011. Data were sourced from a computerized population birth database. The study outcomes were duration of pregnancy expressed as total days, and the difference between aDOB and eDOB by dating method.


Only 5% of births occurred on the eDOB, regardless of the dating method or timing of the dating. Approximately 66% of births occurred within 7 days of the eDOB, and there was little difference among the ultrasound examinations performed at varying gestational weeks. The ultrasound scans at 110–140 weeks of gestation performed as well as ultrasound scans conducted at other gestational ages.


On a population basis, there were no meaningful differences in the prediction of date of birth by ultrasound scan date. An early dating scan (≤ 10 weeks) is unnecessary if LMP is reliable.


Amina Z. Khambalia, Christine L. Roberts, Martin Nguyen, Charles S. Algert, Michael C. Nicholl, Jonathan Morris (2013).  Predicting date of birth and examining the best time to date a pregnancy, International Journal of Gynecology & Obstetrics, 123(2): 105-109, http://dx.doi.org/10.1016/j.ijgo.2013.05.007.



photo credit: calendar by oschene via photopin cc

Want to stay up to date and get regular updates on birth-related research and thinking sent free to your inbox?

Sign up to my newsletter list.

Books to help you become informed and make the decisions that are right for you…

If this blog post or any of my work has helped you or those you care for, please buy me a coffee and help me continue to provide independent birth information online as well as in my books.

Thank you 💜

4 comments for “Women’s LMP dates are just as accurate as scans…

Comments are closed.