Postdates: Separating Fact from Fiction
What is one of the first things that a pregnant woman hears once she reaches 40 weeks?
“When will your doctor induce you?”
Is there evidence behind this practice to support the routine induction of pregnancies that go beyond 40-41 weeks? What are the usual assumptions and beliefs surrounding this?
• There is a higher risk of the baby being born still
• The placenta will stop functioning
• There will be a decrease in amniotic fluid
• The baby will grow too large
We are going to take a look at the validity of these claims and beliefs, and compare them with what the research has to say. After all, your doctor would never do anything that wasn’t in your or your baby’s best interest, correct?
The first things to really look at are the definitions of the two key words with the pregnancy that goes past 40 weeks. Postdates, and Postmaturity. But is it accurate to start with these terms at 40 weeks?
• Postdates – Defined as a pregnancy that goes beyond 42 weeks, based on LMP. The problem with this is that it’s not the same for every woman. Due dates are calculated depending on LMP, but does not usually take into account a woman who has shorter or longer than 28 day cycles. The pregnancy wheel that is commonly used by doctors and midwives, is based on 28 day cycles. If you have a longer cycle, days will need to be added to your EDD ( Estimated Due Date ). This is rarely done however, and women who have longer cycles are held to the same due date estimation as women with shorter cycles. So on paper, you might be 42 weeks according to the estimated due date, when in actuality you would only be 41 weeks. A more accurate way of dating pregnancy is by solidly known conception dates.
• Postmaturity – Postmaturity, or Postmaturity Syndrome (PMS) can only be diagnosed after delivery and is defined as a postdates pregnancy accompanied with a combination of the following newborn assessments:
a) No lanugo ( fine body hair )
b) Long nails
c) Abundant hair on head
d) Calcified fetal skull
e) Hanging or wrinkled skin, with the appearance of weight loss
g) Peeling skin
Postmaturity Syndrome also only affects less than 10% of pregnancies that go beyond 43 weeks. The vast majority of pregnant women do not go beyond 42 weeks with correct dates. Some studies show that less than 3% of women go beyond 43 weeks. So if the risk of postmaturity is less than 10% of pregnancies that go beyond 43 weeks, and the percentage of women who go beyond 43 weeks is less than 3% - how big of a risk is it really?
The problem with assessing risk for postmaturity is that modern Obstetrics, and even modern Midwifery, tends to treat all women as equal in pregnancy. Seldom is personal or familial gestation history taken into account, or abnormal cycle and ovulation schedules. These things are important to consider! How healthy would a midwife’s policy of inducing at 41 weeks , be for a woman who has a personal or familial history of going to 44 weeks? We are talking about potentially trying to induce a baby who will be 3 weeks “early” according to their own biological gestation clock. And if the induction “fails”? It will likely result in stress for both mother and baby and lead to more invasive intervention, and possibly a cesarean.
The condition of a baby and placenta all depends on the health and personal history of the mother, as well as the health of the baby – at any gestation. A placenta does not begin to deteriorate automatically beyond 42 or 43 weeks. A placenta can begin to deteriorate at 36 weeks, once again, depending on the health and over all well being of the mother and baby. I have often heard the fear in women of “placental deterioration” after 40 weeks. But as it has been seen, this has nearly nothing to do with length of gestation, as much as it has to do with overall health and maturity of the individual pregnancy and baby. I personally have seen a baby born at 43 weeks, solid dates, absolutely covered in vernix and attached to a very healthy placenta. In contrast, I attended the birth of a 37 week baby who had dry, wrinkly skin, and a calcified and very old looking placenta.
Other important factors include unhealthy habits and complications such as:
• Diabetes ( Mellitus, NOT Gestational )
When did 40 weeks become the magical number?
The interesting part in the discussion of postdates, postmaturity, and all that it involves, is the thought that 40 weeks is some sort of magical number. In the past, there was a general “due month”. Women were given an estimation of when they would deliver, based on the known fact that normal gestation is anywhere from 37 to 42 weeks. So when did 40 weeks become this magical number that women fret over and worry once they go beyond it? It has always been that 40 weeks is the general time frame when babies were “due”. But it wasn’t until a study by McClure-Brown came out with date collected from 1958, that showed the perinatal mortality rate doubled from 40 weeks to 42 weeks – from 10/1000 to 20/1000. So it might be logical to assume that inducing labor before 42 weeks would cut back the risk of stillbirth, correct?
The problem is, this study is inaccurate and too old to continue to be of use. Modern obstetrics contradicts the findings in the study published in 1963. And yet, the findings continue to be cited. If we accepted the outcomes in the McClure study, we would also have to accept a 10/1000 mortality rate at 40 weeks! And we know that is not correct. We know that in the 1950s, the majority of women were put under general anesthesia, or twilight sleep, and forceps were commonly used.
Modern Obstetric research actually shows there to be not much of a difference in perinatal mortality rates between 38 and 42 weeks, with a decline in between.
An identically set-up chart to the 1963 study, published in 1982 ( Williams, Creasy ) reads:
• 7/1000 at 38 weeks
• 6/1000 at 40 weeks
• 8/1000 at 41 weeks
• 9/1000 at 42 weeks
• 10/1000 at 43 weeks
• 11/1000 at 44 weeks
A graph from 1987 statistics ( Eden, Sefert ) shows:
• 6/1000 at 38 weeks
• 2/1000 at 40 weeks
• 2.3/1000 at 41 weeks
• 3/1000 at 42 weeks
• 4/1000 at 43 weeks
• 7/1000 at 44 weeks
So according to the second set of statistics gathered above, women were at higher risk of stillbirth at 38 weeks, than they were at 42. Interesting! In the first set, there was only a steady increase, resulting in a very small risk increase. Is the slightly increased risk worth the myriad risks that come with labor induction?
A large study done by Weinstein, et al. , compared nearly 1,800 reliably dated post-term pregnancies with a matched group of on-time deliveries ( between 37 and 41 weeks ). The outcomes were surprising. Perinatal mortality was similar in both groups ( 0.56 / 1000 in the post-term and 0.75 / 1000 in the on-time group ). The rates of meconium, shoulder dystocia, and cesarean were almost identical. What was most interesting, however, was that the rates of fetal distress, instrumental delivery and low Apgar scores were actually lower in the post-term group than in the on-time group.
What about the Amniotic Fluid?
There is a flawed belief that the amniotic fluid will somehow begin to “run out” beyond 40 weeks. There is a belief that women will have a “dry” birth. Let’s start with some basics.
What is amniotic fluid?
• Beyond 36 weeks, amniotic fluid is comprised of mostly fetal urine. When the baby’s kidneys are functioning properly, the baby will continuously produce and process amniotic fluid. The fluid is swallowed by the baby, and then urinated out, once processed by the kidneys.
As long as the mother is adequately hydrated, and there are no congenital abnormalities in the baby, the baby will continue to create amniotic fluid until birth. Whether this be at 37 weeks, or 44. If decreased amniotic fluid is suspected through palpation, an ultrasound can be done to measure the volume found. However, this is not an exact science, as the volume found can – and usually will – vary from ultrasound technician to ultrasound technician, and can also sometimes be dependent on baby’s position. If the levels are found to be on the low side, evidence based protocols suggest having mom orally re-hydrate and return within 24 hours for another AFI ( Amniotic Fluid Index ), preferably by a different technician. This has shown repeatedly to have improved outcomes, versus immediate induction for low AFI levels.
A study published in the Journal of Reproductive Medicine found a significant increase in amniotic fluid after maternal oral rehydration, as well as intravenous hydration, with neither one better than the other. In all, 62.5% and 44.0% demonstrated improved AFI levels.
What if the baby grows too large?
First, who defines “too large”? What is “too large” for one woman, might be the next woman’s smallest baby size. The most important thing to remember is that there is no fool proof way of knowing whether or not your body can naturally birth a baby of whatever size, until you have tried. Ultrasound has a 20% error rate in either direction, and many women have allowed an induction after being told that their baby would be nearly 10 pounds, only to give birth to an 8 pound baby. And, there is no reason for a woman to doubt her ability to birth a 10 pound baby unless she tries. I, for one, never would have believed that I could have birthed my nearly 11 pound baby, especially because I was told that I could not safely birth my 8 ½ pound baby that I was scared into a cesarean with. You never know until you give it a full chance.
Women are often told that a baby will gain approximately a ½ pound per week in the end of pregnancy. However, this is simply an approximation. Once again, this is NOT the same for every woman, or for every baby.
According to a fetal growth rate chart comprised by four studies , a baby will only put on approximately 0.56 pounds – that’s just over half of a pound – between 40 weeks and 43 weeks. And since we’ve shown that most women go into labor before 43 weeks, it can be assumed that it is even less than that. Babies hit a plateau with weight gain around 40 weeks. So really, is there a huge concern to be had over a baby being birthable at 40 weeks, but not at 42 if we’re talking about less than half of a pound? And, does less than half of a pound change the shoulder width or head size of a baby? Hardly. It may give baby chubbier cheeks, or chubbier buns, but will not change the overall structure of the baby, making baby automatically “too large” to birth between those two weeks.
When Should Monitoring a “Post Dates” Pregnancy Begin?
This may be different for each individual pregnancy, each individual woman, which makes cookie cutter policies surrounding post dates, arbitrary. To begin, we have now shown that according to research, doctors, and all basic “rules” that a pregnancy is not even considered postdates until after 42 weeks. Not 40. So if the pregnancy is not postdates until 40 weeks, why do doctors often begin Non-Stress Tests ( NSTs ), Biophysical Profiles ( BPPs ), and Amniotic Fluid Index ( AFI ) at 40 weeks? It goes back to the very flawed study from 1963.
It is up to each individual woman to decide if she is comfortable waiting on monitoring, but if a woman understands that there is virtually no risk difference from 38 weeks to 42 weeks, it should clarify that testing before 42 weeks is mainly unnecessary unless other pregnancy complications are present (i.e. Hypertension, Diabetes Mellitus, IUGR suspicion, Congenital Abnormalities ).
So, let’s take a look at what type of monitoring is available, and how effective they are in finding possible problems.
• Biophysical Profile ( BPP ) – A BPP checks fetal body tone, fetal movement, amniotic fluid volume, and fetal “breathing” practices. Each of these are given a score, and then it is added up to give an overall score. A high score of 8-10 usually shows a baby in good health, while a baby who scores 0-4 indicates a baby who needs to be more closely monitored, or needs to be outside of the womb. Scores in between will usually come with more monitoring, including another BPP within 24 hours.
According to Enkin et al., in A Guide to Effective Care in Pregnancy:
There is some evidence that these tests can detect pregnancies in which there is 'something wrong,' but less evidence that their use improves outcome, or can eliminate the additional risk of post-term pregnancy. The only controlled trial shows no advantages of complex fetal monitoring with computerized cardiotocography, amniotic fluid index, assessment of fetal breathing tone, and gross body movements over simple monitoring with standard cardiotocography and ultrasound measurement using maximum amniotic fluid pool depth.
So as you can see, even the detailed testing may not prevent issues that may arise.
According to several studies that researched the accuracy of the BPP, the false positive rates were quite high, resulting in unnecessary induction or further monitoring.
One in particular showed a 21.3% false positive rate for the BPP, and a 39.3% false positive rate for the Non-Stress Test ( NST ). More studies have shown much higher false positive rates for the Non-Stress Test, which is the most common for women who go beyond 40 weeks in care under an Obstetrician.
• Amniotic Fluid Index ( AFI ) – An AFI is basically a mini Biophysical Profile. It measures the maximum amniotic fluid pool depth in the uterus. However, as was shown in the beginning of this article, the AFI in a pregnancy can be contingent on several factors. Being dehydrated can lessen the AFI found. The baby’s position can affect how much amniotic fluid is seen. The skill of an ultrasonographer can make a difference in the AFI level found.
It was also shown that AFI levels can be improved with maternal oral rehydration. Often in modern obstetrics, this protocol is ignored, and induction is recommended very much against proven evidence.
• Non-Stress Test ( NST ) – The NST is the most commonly used test with women who go beyond 40/41 weeks pregnant, under the average Obstetric care. An NST is electronic fetal monitoring for contractions, fetal heart rate variability, and overall heart rate strength. If a baby is found to be sleeping, stimulation is often used in the form of vibration, a cold drink with sugar ( such as orange juice or soda ), or palpation stimulation.
The NST comes with the highest false positive rates of all of the tests, which is why it has become a controversial test amongst some groups.
Studies have been done that conclude anywhere from a 50%-75% false positive rate on average, sometimes reaching as high as 80-90%. False positives will lead to more testing, more stress, and possibly unnecessary intervention in the pregnancy.
• A pregnancy is NOT “Postdates” until after 42 weeks.
• The risk of stillbirth is nearly a flat line between 38 weeks and 43.
• Amniotic fluid is dependent on maternal hydration, in the absence of congenital abnormalities.
• A baby’s weight virtually plateaus after 40 weeks.
Some things to think about :
• If I am not “overdue” until after 42 weeks, should I allow testing or intervention before this point?
• If NSTs come with very high false-positive rates, is it a test worth submitting to?
• If my baby will not put on much weight within a 3 week period, is it logical to worry about my baby being “too large” within a probable 2 week period?
Please, please always do your own research. Question what you are told - and go study the subject – regardless of whether your OB, Midwife, Family Member, or Friends are the ones giving you the information. Make informed decisions, and take charge of your prenatal care!
McClure-Browne, J.C. 1963. Comparison of perinatal mortality rates versus gestational age through the past three decades. Postmaturity, Am J Obstet Gynecol 85: 573–82.
Journal of Fetal Medicine 1996 Sep-Oct. 5(5): 293-97. Expectant Management of Post-Term Patients: Observations and Outcome. Weinstein D. et al.
Journal of Reproductive Medicine 2000 volume 4 pp 337-340. Effect of Oral and intravenous hydration on oligohydramnios. CHANDRA P. C.; SCHIAVELLO H. J. ; LEWANDOWSKI M. A. ;
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