Post Dates in Pregnancy

If we look at the research on the topic of post dates pregnancy for example it can be very confusing. This is mostly due to the uncertainty of dating. The exception is with IVF pregnancy, where the date of fertilization is known.

There are a number of terms women and health professionals use to describe post dates pregnancy. Including prolonged pregnancy,  post term and overdue.  Medically the term ‘post dates’ is often used. This term is confusing as it implies by the name that the reference is the expected date of confinement (EDC). Which at this point and 2 weeks beyond is still considered term or within the normal duration of pregnancy.
Term pregnancy is defined as 38-42 weeks gestation. Thus, post dates or prolonged pregnancy can be considered beyond the 42nd week.
Giving women an expected date of confinement (EDC) requires education that there is a 2 week window or longer either side of that date to consider. This may seem obvious to the health professional and may not be considered as important information. This is particularly relevant to the first pregnancy as a higher percentage will birth beyond 41 weeks.  Only 3% - 4% of women birth on the ‘due date’.  Some countries give a 2 to 4 week range of dates rather than a specific date. I imagine this would decrease the anxiety some women have regarding passing their ‘due date’. 

The commonly used pregnancy dating methods are LMP (last menstrual period) and first trimester ultrasound. For women who choose not to have an ultrasound LMP will be used. Judy Slome is a midwife practicing in Israel who has spent years reviewing and writing about the research on post dates . The following is an extract from one of her papers “Reducing Inductions: Lack of Justification to induce for post dates” . 
"Ultrasound is inaccurate for dating because:

  1. Symmetrically large or small fetuses do not fit the norm.

  2. Ultrasound reference values are not accurate (Lynch and Zhang 2007)

  3. Fetal position affects measurement. Crown-rump length (CRL) measurements are strongly dependent on the fetal position. Biparietal diameter (BPD) measurements are considered by many to be more reliable than CRL, sometimes producing dates within a range of -8 or +8 days from the real date of IVF pregnancies (Sladkevicius et al. 2005)

  4. User error, experience, motivation and talent

  5. Data entry errors"

The literature around medical induction for post dates versus waiting ‘expected management’ is also conflicting. Approximately 10% of women will have induction for post-dates. Induction protocol for post-dates was adopted to prevent stillbirth. The increased possibility of stillbirth as the pregnancy progresses appears to be agreed on in the literature, however, the risk is very low. This is no comfort of course for families that have experienced a stillbirth.
The question to be asked is what is the balance of risk between medical  induction for post dates versus waiting? The risks associated with induction can also not be ignored some include:

  • Amniotic fluid embolism

  • Increased risk of caesarean section

  • Fetal distress

The risk of stillbirth in a subsequent pregnancy for a woman who has had a previous caesarean is also higher. This appears counterproductive to induce to prevent stillbirth which may end in a caesarean which will further increase the risk of stillbirth in the next pregnancy.
Judy Slome Cohain mentioned a curious contradiction in her paper: "The willingness to enact a protocol, such as induction for post dates without any evidence of its long term safety to mother and child”.
Women’s bodies are far to often looked at as faulty in relation to birth. Taking action on this premise in the form of intervention that may not look at the balance of risk to the mother and baby in an otherwise healthy pregnancy may have serious implications for both the physical and emotional long term health of the mother and child.
Written by Julie Uren, Chiropractor
1. Hollis B. 2002. "Prolonged pregnancy." Curr Opin Obstet Gynecol. 14 (2):203–7.
2.  Wennerholm UB, Hagberg H, Brorsson B, Bergh C. Induction of labor versus expectant management for post-date pregnancy: is there sufficient evidence for a change in clinical practice? Acta Obstet Gynecol Scand. 2009;88(1):6-17.
4. Reducing Inductions: Lack of Justification to induce for postdates, By Judy Slome Cohain