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What percentage of stillbirths are due to prolapsed cord and tight knot?
After an extensive search, I wasn't able to determine the percentage of stillbirth due to prolapsed cord or tight knot specifically, however, I found that stillbirth due to umbilical cord accidents is about 10%. I wasn't also able to determine prevalence before or after 27 weeks for either prolapsed cord or tight knot, however, incidence of true tight knot is between 0.3% and 2%, while that of prolapsed cord is between 0.1% and 0.6%. I have completed the spreadsheet and detailed my findings below.
FINDINGS
After searching through articles in academic databases, I wasn't able to find the percentage of stillbirth due to either prolapsed cord or tight knot. Although we generally do not use sources older than 2 years in our research at Wonder, I had to rely on older studies because of the paucity of studies on the subject. Studies on the subject generally focus on umbilical accidents as a collective group and the data is not further segmented. Stillbirths due to umbilical cord accidents, which includes stillbirth due to prolapsed chord and tight knots, is about 10%. Another studies put the stillbirth rate due to umbilical cord incident at between 3.4% and 15%. However, evidence based on autopsy attribute only 2.5% of stillbirths as due to umbilical cord accidents.
The reason specific data for stillbirth due to any of the umbilical cord accidents, such as true knots and prolapsed chord, are unavailable may be due to the fact that there are discrepancies in data on the percentage of stillbirth due to umbilical chords in general. Another reason according to researchers is that umbilical cord related stillbirth "attribution may only reflect inadequate diligence in identifying explanatory familial, medical or histological associations. In order to convincingly attribute a stillbirth to cord accident, it is necessary to demonstrate cord occlusion, hypoxic tissue injury on autopsy, and exclude other accepted causes of stillbirth. These conditions are rarely met. Taken together with the frequency of nuchal cords in normal pregnancies, the actual proportion of stillbirths due to cord accidents remains uncertain."
In terms of the prevalence before or after 27 weeks, I wasn't able to find such data. This is probably because it is still very difficult to diagnose true knot or prolapsed cord prenatally and most discoveries are incidental. Studies on the incidence of true knots estimate it to be 0.3-2%. For prolapsed cords, it is about 0.1-0.6%.
ADDITIONAL HELPFUL FINDINGS
I was able to find a couple of useful data specific to true knot and prolapsed chord. According to a study on true knot, "there was a four-fold higher rate of antepartum fetal death among those fetuses (1.9% versus 0.5%, P<0.0001)" and the delivery was more often than normal through "a cesarean section (130/841 versus 711/68,298, P<0.0001)."
In a study of 13 true knot cases found during a study of 961 deliveries, 8 were discovered at birth with no clinical significance, while 4 caused signs of fetal distress that prompted a cesarean section. The only death occurred at the 19th week of gestation (before the 27th week you are interested in).
There weren't similar studies on umbilical cord prolapse but its manifestation is generally treated as an emergency. A large study found prolapse cord mortality rate to be 91 in every 1000 births.
GRADE OF MACERATION
As requested, I looked into Grade of Maceration data for both umbilical prolapse chord and true knots. Unfortunately, there are no published studies on that available and information on Grade of Maceration in true knots and umbilical cord cases are sparse.
As you mentioned, grade of maceration can be used to estimate time of death, but such studies have not been carried out specifically to determine the time of death in true knot or umbilical prolapse stillbirth cases.
STANDARD OF CARE
For prolapse chord, when it is diagnosed before full dilation, it is treated as a medical emergency and preparation is made for immediate delivery at a theater. Cesarean section is also recommended when vaginal delivery is not imminent to prevent hypoxic acidosis.
It is important to note that prolapse chord diagnosis is rarely diagnosed during antenatal care. Prolapse chord can be diagnosed with ultrasound but it is not sensitive nor specific enough and is not recommended outside a research setting. So prolapse chord presentation is generally discovered when considering vaginal birth.
For true knot, the standard of care is not very clear as the majority of true knots have no clinical significance and are generally incidental findings. For instance, in a study of 967 deliveries, 13 true knots cases were discovered (1.4% prevalence rate), and 8 of the 13 true knot cases were incidental discoveries during delivery with no clinical significance.
If discovered early, the recommended standard of care is "follow-up sonographic imaging and close monitoring until the determination of fetal maturity gives the best chance for a good outcome." When discovered during labor where it is causing fetal stress, a cesarean section is generally recommended. I have also completed the spreadsheet with the requested information.
CONCLUSION
To wrap it up, although there aren't studies available on the percentage of stillbirths due to prolapsed cords and true knots, we found that that stillbirth due to umbilical cord accident is about 10%. Incidence of prolapsed cord is about 0.1-0.6%, while that of true knots is between 0.3-2%. I have also filled the spreadsheet as requested.