Due date estimation guidelines

Compelling evidence shows that the most accurate estimation of when a normal pregnancy (gestation) will end is the projected date yielded by the mostly routine first trimester ultrasound (up until 13 weeks, 6 days).

As it turns out, the old-school estimated date of delivery (EDD), worked from the last normal menstrual period (LNMP), correlates very well with this ultrasound estimate – providing that accurate information about a woman’s LNMP is available. The LNMP EDD is worked out as follows:

  • Take the date the last normal menstrual period started (LMP) – e.g. 3 March 2019
  • Add nine calendar months to this date – e.g. 3 December 2019
  • Add seven days and you have an expected due date – e.g. 10 December 2019

Gestational term classification

When correlating data on birth outcomes, both positive and negative, using the concept of ‘term gestation’ as 37-42 weeks (as has been adhered to for decades), has presented many challenges in the past. For the purpose of uniformity, data collection and reporting, the following useful recommendations for classification of gestational term from 37 weeks of pregnancy have been made (ACOG, 2019), and prove to be far closer to physiological labour and birth norms:

  • Early term: 37 weeks, 0 days – 38 weeks, 6 days
  • Full term: 39 weeks, 0 days – 40 weeks, 6 days
  • Late term: 41 weeks, 0 days – 41 weeks, 6 days
  • Post-term: 42 weeks, 0 days and beyond


  • High quality first trimester ultrasound should be done to establish or confirm gestational age and to calculate the EDD. The LNMP method should be used very circumspectly.
  • If pregnancy resulted from assisted reproductive technology (ART), an EDD from the ART-delivered gestational age should be assigned.
  • Changes to EDD should only be made in rare cases, and should be documented clearly in maternal records.
  • A scheduled delivery should not take place before a gestational age of 39 weeks. If an accurate gestational age is not available, a scheduled delivery is contraindicated.
  • Trial of labour is the only way to know for sure whether or not a woman with an uncomplicated pregnancy will be able to birth naturally.


  • Mother Nature is the ultimate midwife. It is best to allow Mother Nature to take her course toward the end of pregnancy, if all is well with mother and infant.
  • It takes patience but this will pay off in an easier birth, a calmer baby and less parenting challenges.
  • First pregnancies often go over the due date. Lightening may occur between 36 and 38 weeks but in many nulliparous women this is not easily noticed. Many babies only engage completely once in active labour, especially in first pregnancies and when a woman has good abdominal muscle tone.
  • Encourage women not to work right up to the very end if possible, but to use these last weeks to rest and relax, take part in moderate exercise like walking, dancing or swimming, to spend time with their partners, family and friends, and to prepare mentally for mothering.
  • Advise women to stick to regular check-ups to ensure that both she and Baby are doing well.
  • Although fetal movement might slow a little, normal patterns of movement should still be recognised and any abnormalities investigated.
  • Suggest warm baths to help her cope with Braxton Hicks contractions.


  • ACOG (The American College of Obstetricians and Gynaecologists) Committee on Obstetric Practice & Society. 2013. Reaffirmed 2019. Definition of Term Pregnancy. [Pdf]. Available at: https://www.acog.org. Accessed September 2019.
  • ACOG (The American College of Obstetricians and Gynaecologists). 2013. ACOG Committee Opinion No. 561: Nonmedically indicated early term deliveries. Obstetrics & Gynecology, 121(4):911–915.
  • ACOG (The American College of Obstetricians and Gynaecologists), American Institute of Ultrasound in Medicine (AIUM) & Society of Maternal-Fetal Medicine (SMFM). 2017. Committee Opinion No. 700: Methods for estimating the due date. Obstetrics & Gynecology, 129(5): e150–e154.
  • ACOG (The American College of Obstetricians and Gynaecologists). 2017. ACOG Committee Opinion No. 688: Management of suboptimally dated pregnancies. Obstetrics & Gynecology, 129(3): e29–e32.
  • Caughey, A.B. 2015. The safe prevention of the primary caesarean. Clinical Obstetrics and Genecology, 58(2): 207–210.
  • Cohain, J.S. 2018. More Evidence to avoid hospital birth: a critique on the results of the arrive study. Available at: https://midwiferytoday.com/mt-articles/more-evidence-to-avoid-hospital-birth. Accessed September 2019.
  • Gaskin, I.M. 2003, Ina May’s Guide to Childbirth. Bantam Books, New York
  • King, T.L. & Pinger, W. 2014, Evidence-Based Practice for Intrapartum Care: The Pearls of Midwifery. Journal of Midwifery & Women’s Health, 59(6):572-585.
  • Marshall, J. et al (eds). 2016. Myles Textbook for Midwives (African Edition). Elsevier Ltd, Oxford, UK
  • WHO (World Health Organization). 2018. WHO Recommendations: Intrapartum care for a positive childbirth experience. [Pdf]. Available at: http://apps.who.int. Accessed September 2019.
  • Wilmink, F.A., Pham, C.T., Edge, N., Hukkelhoven, C.W.P.M. Steegerd, E.A.P. & Mol, B.W. 2019. Timing of elective pre-labour caesarean section: A decision analysis. The Australian & New Zealand Journal of Obstetrics & Gynaecology. 59(2):221-227.