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Pregnancy Bliss | Reproductive Health Hub

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For a scan performed around twenty weeks, if the discrepancy between the mother's dates and the scan is less than ten days, the mother's date is usually given the benefit of the doubt. If it is more than ten days, she will be advised that the estimation from her date is very likely wrong and therefore the expected date of delivery is revised, using the scan estimate as the basis. It is usually a point of contention because dates are, in most cases, reviewed backwards, putting the expected date of delivery further ahead. Nobody wants to carry a pregnancy for a day longer, if this can be helped! However, this may be crucial, for reasons discussed earlier.



Can a twenty weeks scan predict the eventual weight of the baby at birth?

No.

It is not possible at twenty weeks to accurately predict whether there will be restricted or excessive growth. If the fetus is found to be abnormally small at this gestation, and if there is no reason to doubt the accuracy of the gestation, this signifies a more profound problem and not simple growth restriction. Such a finding calls for further tests. If the fetus appears larger than it should be at twenty weeks, as a rule, it means the gestation is wrong. It does not mean the fetus is growing into a big baby.



Occasionally there is a big discrepancy between the gestation as estimated by abdominal examination and that estimated by ultrasound scan. Which one should be trusted?

This problem is uncommon but by no means rare. In such a case, the scan is the one which should be taken to be accurate. This is because the scan measures the fetus itself, while abdominal examination depends on the size of the womb (fundal height). The latter may be influenced by several unrelated factors such as a full bladder, amniotic fluid volume, pelvic tumour or maternal obesity.

Scan accuracy (for dating) holds true only in the first half of the pregnancy. This is why it is essential to establish the accurate date in this phase of the pregnancy. Both the scan and abdominal examination could be grossly inaccurate and wildly off the mark in late pregnancy.

For late-bookers, there are occasions when it is actually impossible to accurately date a pregnancy.


How much of the scan findings at an ultrasound scan are documented?

Normally the person performing the scan will produce a report that highlights all the significant findings. Whenever it is seen fit, still pictures may be taken for future reference. Sometimes, a video recording is made sometimes to allow for consultation with colleagues who may not be immediately available during the test but also for training purposes. A verbal or written consent is usually obtained from the mother for this.



How comfortable is the test?

Probably the only discomfort encountered by the mother is the full bladder that she is asked to maintain for the test in early pregnancy. A full bladder is really important because it facilitates the lifting of the womb out of the pelvis into the abdominal cavity. This allows for optimal scanning. If the bladder is empty, scanning is possible but may be incomplete, because some parts of the fetus may be deep in the pelvis and it may not be possible to see them properly or take measurements. Also, a full bladder facilitates good scan views that are helped by the water in the bladder directly overlying the uterus.


A full bladder is not required for a vaginal scan; the mother will be asked to empty her bladder before the procedure.

The expectant mother at a routine eighteen to twenty weeks scan should be assured that she will lie on a comfortable couch or bed, well propped up to see the screen, and she won't have to take her clothes off. The test lasts twenty to thirty minutes only, unless there is an unforeseen problem.


Findings of the pregnancy scan


What exactly will the examiner be looking for during scanning?

In summary, the number of fetuses will be confirmed, the viability of the fetus, anatomy of the fetus, placental anatomy and its location, fluid volume, the size of the fetus and a specific search for particular abnormalities.


The report for every scan will mention the presence or absence of gross abnormalities, adequacy (or otherwise) of fluid volume, the placenta (location and any abnormality) and the size of the fetus. The opportunity is used to assess the other pelvic structures, especially the ovaries. Any significant finding will be documented.

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Early trimester Placenta and fluid 20 weeks scan Safety Uses in pregnancy Limitations 4D scans Fetal gender