The Surprising Lack of Evidence For Postdate Birth Induction

The Shocking Lack Of Scientific Evidence For Inducing Birth

Abstract: Case controlled studies show no benefit to inducing for postdates. Routinely inducing for postdates is based on 3 uncontrolled, retrospective studies showing 1/1000 less stillbirths while ignoring possible increases in brain damage to newborns as a result of induction. Induction or Elective cesarean for Precious pregnancies is justifiable.

Judy Slome Cohain, CNM

There is a low risk of stillbirth at ≥42 weeks in the US although it is even lower in Europe.  Once you get to term, the chance of a healthy woman delivering a live baby is 99.95% in Europe and 99.90% in the US. If post-term induction were to be shown in the future to have the capability of preventing stillbirth without increasing brain damage and perinatal mortality, about 2,000 healthy, low risk women would have to be induced at 40 weeks to prevent one stillbirth.

Stillbirth and Neonatal death happen throughout pregnancy (1,2).

Term stillbirth and perinatal mortality rates: European rates in red , US rates in blue

Week Stillbirth/1000 undelivered Neonatal deaths/1000 live births
37 1/3000         1/1400 2.7/1000
38 1/2500         1/1250 1.5/1000
39 1/1800         1/1000 0.9/1000
40 1/1300          1/800 0.7/1000
41 1/800            1/750 0.9/1000
42 1/600            1/900 1.3/1000

It is expected that 98% of women do NOT deliver on their due date, but rather 49% before and 49% after their due date, and only 2% on their due date. Since first births on average deliver a week after the due date, about 75% of first births deliver after their due date.  Data from 2002-2008 in the US showed  11.4% of low risk first births and 6.4% of low risk multiparous births that attempted vaginal birth were induced for the reason of  'postdates’ and a total of 45% of births, that were not elective cesareans, were induced.

Inducing for postdates originated from an uncontrolled, non-randomized observational study from 1999 showing a drop of 1/1000 in the stillbirth rate at several hospitals, occurring serendipitously during the same time frame as an increase in the induction and cesarean rates in Canada.  The drop in stillbirths in this type of research can be explained by a change in many other possible variables that took place during the same time period. This result has never been shown by case controlled trials. The best quality evidence says that induction for postdates does NOT reduce the number of stillbirths or improve perinatal mortality rates.

The best quality evidence says that induction for postdates does NOT reduce the number of stillbirths or improve perinatal mortality rates.

Documented Drawbacks of routine induction: High induction rates are associated with an increase in premature births between 34–36 weeks, increased births of problematic babies that need intensive care,  higher cesarean rates and 1 in 3000 maternal deaths due to cesarean, a possible increase in brachial palsy due to undo force of induced contractions, and two fold increase in amniotic fluid embolism.

There is some logic behind a desire to deliver In Vitro Fertilization (IVF) pregnancies at 40 weeks to avoid the rare cases of stillbirth that take place after 40 weeks. Since you know when fertilization took place, you know when the fetus is fully developed.  IVF babies are particularly precious since they may not be replaceable. There is some logic behind delivering uncontrolled diabetics early to avoid stillbirth for similar reasons. 

The same cannot be said for non-IVF pregnancies. Ultrasound dating is only accurate to the nearest 2 -4 weeks. A woman who thinks she is 40 weeks by 10 week ultrasound, might only be 36 weeks and if induced, the fetus may need intensive care, and therefore would be more vulnerable to problems such as infection.  About  90% of pregnancies currently defined as post term would not be so if instead of rounding out pregnancy to 40 weeks (280 days) it was counted as 284 days.  When 284 days are used to define pregnancy instead of 280, less than 1% spontaneously deliver ≥2 weeks late, rather than 10%. 

If a prenatal test could predict which pregnancies would end in third trimester stillbirth, then doctors could do the test and induce those pregnancies. But, no prenatal test has been shown to predict stillbirth, therefore doctors are only left with the option of inducing all postdate births or none of them.   

First births on average deliver at 41+0 weeks or 287 days.  In the absence of any other explanation for delayed birth in primiparous women, this phenomenon appears to be explained by the increased level of anxiety of women on their first pregnancies. Women who want to finish a doctoral thesis, attend a wedding, or an important event are well known to deliver after their desired action is finished. Women who go past 42 weeks either have incorrect dating, a wedding they don’t want to miss etc., or fears directly or indirectly related to the birth, family or mothering that have not been dealt with.

Having warm care providers available helps prevent prolonged pregnancy and therefore stillbirth. Having one-on-one midwifery model ongoing support in which fears are expressed and dealt with, the mother starts birth informed and knowing that the person they want will be available and attending the birth, and the same stillbirth rates and perinatal mortality are obtained with much lower cesarean rates, as with medical management with routine induction at 41 or 42 weeks.

What is the research evidence

Postdate induction is based on unreliable ultrasound dating. Pregnancy is about 284 days not 280.

The only pregnancies in which the approximate time of fertilization is known are In Vitro Fertilization (IVF) pregnancies.  The fertilization of IVF pregnancies is known to take place somewhere within 18 hours of combining egg and sperm in the Petri dish. This knowledge of fertilization has been used in studies to see how accurate first trimester ultrasound is in establishing due date.

Ultrasound specialists, blind to conception date of IVF pregnancies, were only able to estimate the age of the IVF pregnancies within -15 days and +14 days of the actual known due date in the first trimester. (4)  Among accurately dated IVF pregnancies, if ≥284 days were used to define pregnancy, instead of ≥280 days, in the absence of induction, post term pregnancies would account for less than 1% of pregnancies (4).  That means that if pregnancy is considered 284 days instead of 280 days, less than 1% of women would go beyond 42 weeks, instead of about 10%.

According to the 1980 edition of Williams Obstetrics Textbook, published before the widespread use of labor induction for postdates, 10% of pregnancies persist for 42 weeks or more. In a large study done with the intent to reduce post term pregnancy using ultrasound, the post term rate was reduced to 2.5%(5). However, in a 2010 study from Sweden using a population of over 1,175,000 singleton births from gestational week ≥37, for which 95% had first trimester ultrasounds, 9% of pregnancies still persisted for 42 weeks or more (6).

In a study of 17,000 Finnish women from 1993–98 trying to prove the usefulness of ultrasound dating between weeks 8 and 16, 10% of women went post term. (7). When two days were added to the length of pregnancy from LMP, only 6% of deliveries were post term(7). This is the only study of first trimester ultrasound that also looked at outcomes, and it found no increase in perinatal mortality among deliveries after 42 weeks. 

Research comparing induction to waiting

Stillbirth and perinatal death occur at every week of pregnancy.  BUT induction of labor at any week has not been shown to decrease stillbirth or overall perinatal mortality and may increase maternal and newborn morbidity. The stillbirth rate after 40 weeks is 1/2000 pregnancies(8). Does inducing 2000 women at 40 weeks prevent one stillbirth?  Systematic reviews say NO. Systematic Reviews of the research which were large enough to study perinatal outcomes have found no significant difference in perinatal mortality including stillbirths between induction and expectant management groups(1,9,10*, , and 11).

The studies supporting induction at 41 weeks are 3 low quality observational studies. The first is co-authored by Mary E. Hannah, author of Breech Trial, who persistently presents "data with serious concerns as far as study design, methods, and conclusions with frequent lack of adherence to the inclusion criteria."(12). Hannah’s 1999 article promoting induction for post term linked two things that may not be related: the stillbirth rates in Canada as a whole and the induction rates reported at Canadian hospitals close to where she lives, between 1980–1995(13). Perinatal mortality rates were not examined during the period. Stillbirth rates went down from 3/1000 to 2/1000.

This could have been the result of greater availability of abortion on demand, improved hospital protocols, prenatal care, nutrition, and/or less smoking or other unknowns. During the 15-year study period, induction rates increased or decreased at different rates in each place, but in general increased from 12% to 16%. Cesarean rates also increased. It is not made clear by how much; only small samples of cesarean rates at 40 and 41 weeks in the years 1986, 1992 and 1995 are given (13). Hannah’s study compares induction in one place to stillbirth rates at different places during the same time period. The studies neglect to report on perinatal mortality, and withhold known cases of babies who were permanently injured directly as a result of postdates induction protocol. (14)

The second study is by a group of five Canadian academics who chose to study American, not Canadian, birth certificate data from 1991–1997 (2). Birth certificate data is known to be unreliable. The birth certificate data used shows no indication of why labor induction was used and the authors admit the data includes inductions for an already dead fetus and for a fetus that was already compromised. Therefore, the outcomes of actual inductions involving a live fetus is unknown. Like the other studies, this study compares stillbirth rates (not perinatal mortality) to approximate induction rates during a six-year period in which induction increased from 10% to 20%.

A third study is again retrospective research using birth certificate data reporting a 1/1000 decrease in stillbirths associated with induction.(15)  Again, the data is birth certificate data. Again important variables that affect stillbirth rates are not controlled for.  This study shows that induction is associated with a significantly higher rate of problematic babies that need intensive care though no long term follow up was made to know if this results in a significantly higher rate of long term brain damage.

All 3 studies admit that it is impossible to draw conclusions or base protocol on their findings since so many factors could lower the stillbirth rate. Nevertheless, those studies are all the evidence there is to support a policy of induction at 41 weeks.

Many doctors are opposed to postdate induction protocols

 "The induction of labour between 41 and 42 weeks is a very crude strategy for reducing term and post-term stillbirth rates. Although the risk of fetal death is increased after 42 weeks, many more fetuses die in utero between 37 and 42 weeks than die in the post-term period. It appears that smaller term fetuses run a far greater risk than their larger counterparts, and that current methods of antepartum assessment of the term fetus are still inadequate" (16).   Difficulty in identifying at-risk fetuses is what has led to routine inductions and better methods are needed(17).

Prenatal testing cannot predict stillbirth or perinatal mortality

As much as we would like a prenatal test and subsequent intervention that would prevent postdate stillbirth, there is none. Risk screening and prenatal tests have many false positive results and the majority of adverse outcomes occur in the larger population of women identified as low-risk. "There is no effective screening test that has clearly shown a reduction in stillbirth rates in the general population"(18,19). Tests that do NOT decrease stillbirth and perinatal mortality compared to control group (19) are:

  • Fetal movement counting
  • Routine ultrasound scanning
  • Doppler velocimetry
  • Detection and management of maternal diabetes mellitus
  • Antenatal fetal heart rate monitoring using cardiotocography
  • Fetal biophysical profile test scoring (BPP)
  • Vibroacoustic stimulation
  • Amniotic fluid volume assessment (AFI)
  • Home vs. hospital-based bed rest and monitoring in high-risk pregnancy
  • In-hospital fetal surveillance unit
  • Use of the partograph during labor
  • Cardiotocography during labor with or without pulse oximetry

Macrosomia

In addition to supposed avoidable stillbirth, another false justification for inducing labor postdates is based on the fact that the healthy fetus continues to gain weight in utero and larger babies may suffer damage on the way out. The 2011 Cochrane Review found three trials involving 372 women rigorous enough to draw conclusions. The evidence shows that induction of labor for suspected fetal macrosomia in non-diabetic women has not been shown to alter the risk of maternal or neonatal morbidity (20).

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Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment. Views expressed here do not necessarily reflect those of GreenMedInfo or its staff.

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Postdate Birth Induction



75% of my four children were late, the second one three weeks late. The date of conception was never in doubt. The OB with whom I was working on that second pregnancy assured me that the baby knew when it was ready, and she did indeed — popped out painlessly after barely an hour of labor, a healthy, perfect little girl. Subsequently I stopped worrying over #3 and #4. Everyone around me was in a swivet, but I smiled calmly and said, "The baby knows when it's time." Guess what: they always did.

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