Brace Yourself!!!
I have been reading up about EDDs and Prolonged Pregnancy.
Here is the evidence - I've extracted this form a few source and Guidelines.
Women need to be informed of the possible risks associated with interventions to induce labour, as well as the options of continuing with the pregnancy, in order to make an informed choice.
In pregnancies which continue beyond term (37-42 wks of gestation), the chances of perinatal morbidity and mortality increase slightly (Crowley P: Interventions for preventing or improving the outcome of delivery at or beyond term, the cochrane database of systematic reviews 1997)
There are several studies:
One large observational study in Scandinavia (Bakketeig L, Post term pregnancy : magnitude of the problem. 1989) indicates that the number of perinatal deaths is lowest at 40wks (0.2%), rising to 0.3% at 42wks and 0.4% at 43wks. However to contradict this, there are studies that show that there is little difference in meonatal morbidity, but an increase in length of labour and operative delivery (Alexandra et al: Forty weeks and beyond: pregnancy outcomes by wks of gestation 2000, Caughey et al Complications of term pregnancies beyond 37wks 2004). Two large retrospective studies and one small prospective study have suggested that babies born bayond 42 wks gestation are at a moderately increased risj of neonatal morbidity, including fetal compromise, low Apar score (<4 at 5min), and meconium aspiration. It is worth noting that it is well recognised that meconium staining of the liquor is more common in prolonged pregnancy. However, a systematic review indicates that the risk of meconium aspiration syndrome and neonatal seizures is unaffected by routine induction of labour after 41 wks, indicating that the neonatal morbidity associated with prolonged labour many have another underlying cause. - so all this means that it is not clear whether prolonged pregnancy by itself causes increased perinatal mortality and morbidity, or whether factors that put the fetus at risk also prolong the duration of pregnancy. Better identification of clinically dymature fetuses and those that are SGA (small for gestational age) may enable interventions targeted at those fetuses that are truely at risk.
Two Independent Swedish studies (Classon et al Outcomes of post erm birth: the role of fetal growth restriction and malformation 1999, Divon et al, fetal and neonatal mortality in posterm pregnancy: the impact of gestational age and fetal growth restriction 1998) indicate that the increased risk of still birth in prolonged pregnancy is party explained by and increase incidence of SGA infants. This rose from 2.2% among term births to 3.8% among babies born at 42wks or beyond. So indicating that not all babies are equally at risk, and that AGA (Appropriately grown for gestations age) infants may be better able to cope with prolonged pregnancy.
Calculation of due date and definition of prolonged pregnancy:
Of course all this is dependant on what your dates are, and how "accurate" they are, as none of the methods for estimating EDD are accurate and there are variations in calculating EDD.
Estimates about the frequency of prolonged pregnancy vary between 4 and 14%. But the natural incidence of prolonged pregnancy is difficult to define because of interventions in modern obstetric practice.
There are some correlation between factors such as hereditary, racial, seasonal variations, women who work nights, and male fetuses suggesting that prolonged pregnancy is more common in these groups, as well as some rare major congenital abnormalities.
A systematic review of the evidence indicates that routine early pregnancy ultrasound should be used to predict EDD, in order to reduce the number of women who require induction of labour for apparently prolonged pregnancy. This recommendation is supported by evidence that menstrual dates systematically overestimate gestational age when compared with scans.
Using the last menstrual period (LMP) is the most common method of calculating gestational age but many be inaccurate if periods are irreglar, if conception occurs soon after or during use of oral contraceptives, or if menstrual history is uncertain. Estimations based on LMP use Naegele's rule, a calculation of the menstrual cycle and ovulation. Naegele's rule is based on the precept that pregnancy lasts 280days fro the first day of the last period and assumes a 28day cycle with ovulation occurring on day 14. A study of 400,000 birth in sweeden (Bergsjo et al, duration of human singleton pregnancy: a population-based study 1990) indicated that more accurate estimate of due date would be reached by adding 283, rather than the more usual 280days to LMP.
Fetal well-being and prolonged pregnancy
Several tests are commonly used to asses whether the pregnancy is affecting the well-being of the fetus.
The antepartum carditocograph (CTG) is essentially and assessment of the fetal health at the time of the observation and its ability to predict continuing fetal well-being is therefore limited.
A non-stress antepartum CTG, sometimes described as the non-stress test (NST), in usually the first choice as it is non-invasive and easy to perform. However published randomised controlled trials of antepartum CTG provide no support for this technique, and there is no demonstrable improvement in birth outcomes and its use may even be linked with increased perinatal mortality and is linked with an increased C-section rates.
An estimation of amniotic fliud Volume (AFV), including max pool depth and amniotic fluid index (AFI), form a standard part of fetal assessment where the pregnancy is thought to be prolonged as both fluid volume and index diminish progressively through the latter part of the third tri, The mean AFI in prolonged pregnancy is 10.4am, with the third centile at 5.8cm. An AFI of less than 5cm is generally accepted as a definition of Oligohydramnios. Although Oligohydramanios is not necessarily associated with adverse outcomes, it is associated with intrapartum fetal heart rate decelerations and passage of meconium. Recent research suggests that although AFI is superior to a measure of the single deepest pool of liquor, it has a poor sensitivity for adverse outcome and its routine use if likely to increase obstetric intervention without improvement in the perinatal outcome. (Morris Et al, The usefulness of ultrasound assessment of amniotic fluid in predicting adverse outcome in prolonged pregnancy 2003)
Induction of labour versus expectant management for prolonged pregnancy
there is considerable controversy surrounding the management of prolonged pregnancy, based on the uncertainly about the degree of fetal risk and disagreement over the need for medical intervention.
One meta-analysis indicate that routine induction of labour between 41 and 42 weeks reduces the risk of perinatal death in normally formed babies.
Another trial with women with uncomplicated pregnancies at 41 weeks was unable to demonstrate a significant difference in perinatal or maternal outcome and concluded that either management policy was acceptable. (National institute of health and human development network of maternal-fetal medicine units. A clinical trial of induction of labour versus expectant management of post term pregnancy 1994)
A similar study, sought to asses the risks of maternal and perinatal morbidity concluded that the decision to intervene at 41 weeks on the basis of gestational age alone does not appear to be justified, but recommended that beyond 42weeks induction is still the preferred management option (Roach et al, pregnancy outcomes beyond 41 wks gestation 1997)
A large USA review concluded that routine induction of labour at 41wks is likely to increase labour complications and operative delivery without significantly improving neonatal outcomes (Alexandra 2000)
Induced labours are associated with higher rates of further interventions such as instrumental delivery and c-section and with social economic costs resulting from injury and longer stays in hospital. One study has shown that routine induction is less expensive than the alternative of serial monitoring in prolonged pregnancy (perhaps a worrying motivation!) (Hannah et al postterm pregnancy: putting the merits of a policy of induction of labour into perspective 1996).
It has also been demonstrated that, although induced labour is associated with a higher rate of c-section than spontaneous labour following a policy of routine induction at 41 wks. It has been pointed out that while induction itself may be the factor, it is more likely that the indication for induction in the group is the cause of the higher rate of c-section. (Roach 1997). A cohort study concluded that this increased intervention rate can be explained by the clinicians' heightened vigilance, where this resulted in the lower threshold to intervene in a pregnancy perceived to be "high-risk" (Luckas et al Comparison of outcomes in uncomplicated term and post-term pregnancy following spontaneous labor 1998)
Summary = if you have managed to get through all that lot!!!
- A policy of performing a routine early ultrasound to predict EDD appears to reduce the number of women who require induction of labour. However, if current practice were amended in the light of research, and 3 days were added tot he formula, LMP might prove to be just as accurate as ultrasound in calculating EDD.
- While studies consistently demonstrate a rise in morbidity and mortality rates with advancing gestation, perinatal deaths are rare and the actual risk of either remains small.
- Although increased fetal surveillance can provide reassurance to both parents and clinicians in prolonged pregnancy, there is still no clear indication for the most effective method of testing, nor when testing should be initiated.
- A policy of routine induction between 41 and 42 wks gestation reduces the risk of perinatal mortality, but as least 500 inductions need to be done to prevent one perinatal death. It is not clear whether women feel this is an acceptable increase in intervention, particularly when compared to the alternative of increased fetal surveillance.
- Women need to e give accurate, up-to-date unbiased information about the risks of prolonged pregnancy, the options for elective induction of labour or expectant management and increased fetal surveillance and the poential benefits or risks of these.
Gold stars to anyone that reads all this!!
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