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Pregnancy Bliss | Reproductive Health Hub

Estimating fetal weight using ultrasound

Comment: Hi, I've heard that late pregnancy scans has an error of about 1kg. Is that true? I had a disastrous second birth experience as I had so many scans in the last month of the pregnancy and all the consultants said that my baby is very small, just over 2kg and there for they have force me to have another C-section. However my baby girl was born completely healthy and her weight was 3.2KG, which indicates that I have been given totally incorrect information as this error in late pregnancy scans was completely disregarded. I would appreciate as much information about late pregnancy scanning errors as possible. Thank you very much in advance. Kind regards. G. (UK)

Answer: There are two issues here. One is the accuracy of the ultrasound fetal weight estimation in late pregnancy and the other is deciding on mode of delivery on the basis of estimated fetal weight. It is indeed the case that fetal weight estimate using ultrasound in late pregnancy is not particularly accurate. The quoted margin of error is 15-20%. It means, therefore, the weight could be over-estimated or under-estimated by such a margin. That should always be borne in mind when giving advice. However, I’m rather puzzled by the apparent huge error in your case. Estimating 2kg for a baby that turns out to be 3.2kg is far bigger than would be expected. That is very unusual.

On the question of mode of delivery, the decision to advise you to have a caesarean section is also rather puzzling. It appears you had a caesarean section with your first baby. Your obstetrician would have been trying to make sure that it would be safe for you to have a vaginal delivery in this subsequent pregnancy. If there was concern about the baby’s weight, it is reasonable to use ultrasound to estimate the weight. Normally, this is combined with the clinical impression i.e. how big the baby feels on examination. If the baby weight was estimated to be significantly above average, it would be legitimate to consider a repeat caesarean section the preferred mode of delivery. However, if the baby’s weight is estimated to be low (as was the case with your baby), it is difficult for me to understand why a vaginal delivery was not to be attempted. Unless there was another issue that was considered to be an additional risk factor, a previous caesarean section and a baby of low weight does not preclude vaginal birth.

Ruptured membranes

Question:  Can you please explain to me a ruptured membrane during labour? What would be the cause? How serious is it? M.S. (UK)

Answer: Membrane rupture is simply the medical term for the breaking of waters. It is not a complication. When waters break by themselves that is termed ‘spontaneous rupture of membranes (SROM)’. When waters are broken deliberately by the midwife or doctor as is sometimes done on inducing labour or during labour itself, the term is ‘artificial rupture of membranes (ARM)’. It is a normal part of the labour process.

Prolonged pregnancy up to a year?

Question:  Are there pregnancies that last up to a year? What are the causes and chances of child survival. Thanks. S.B. (Kenya)

Answer: The probability of a pregnancy being prolonged up to a year and the baby surviving is so small that, it can safely be discounted.It is true that in the literature there is a story of one American woman named Beulah Hunter who is supposed to hold the record for the longest pregnancy ever. This is supposed to have lasted 375 days so, well over a year. However, this is from the 1930s and reliability of the report is extremely suspect. Many now believe that this woman would have indeed been pregnant, probably miscarried early on and proceeded to conceive again within a month or so thereby leading doctors to believe it was the same pregnancy, just abnormally prolonged. Remember, these were days when there was no ultrasound scanning.

The placenta, on which the fetus is wholly dependent, has got a strictly limited functional life-span. It is very difficult to see how it can support a fetus for 3 or 4 weeks beyond its natural life-span which is the 42 weeks of human gestation. I suppose this is something that will be very difficult to prove because intervention is always made whenever a pregnancy appears to be going beyond 42 weeks and certainly by 43 weeks, every baby will either have arrived spontaneously or been delivered otherwise. This is a subject we have covered in considerable detail here:

Vaginal ultrasound and hymen integrity

Question:  Does ultrasound break virginity to virgins if performed to their reproductive system? K. (USA)

Answer: The hymen is a band of tissue that can be broken relatively easily by anything inserted into the vaginal canal. The ultrasound probe (see image below) used to perform this test can indeed break the hymen. In many cases where the vaginal approach is required to get clearer details, if the woman is a virgin, she could be offered to have the ultrasound performed rectally whereby the probe is inserted in the back-passage.  This is, in fact, the approach used for children when this is deemed necessary.

This alternative approach, if acceptable to the woman, will give results which are just as good. However, in many cases, the vaginal approach for a pelvic ultrasound is not  absolutely necessary as the abdominal approach is sufficiultrasound vaginal probeent to get all the required information and therefore the issue does not arise..