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Pregnancy Bliss | Reproductive Health Hub

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How is gestation determined on the scan?

This is by taking various measurements of the head, body (abdomen), thigh-bone (femur) and occasionally (only when necessary), the size of part of the brain (cerebellum) or even the length of the foot.

Can a twin be missed at a twenty weeks scan?

This is virtually impossible.

If a mother is told she is carrying one baby at twenty weeks, she should expect no surprises later on. Likewise, if the count is two at this stage, there is no chance it is going to mysteriously be three later on.

How often is a major fetal abnormality discovered at a twenty-weeks scan?

In about 2-3 per cent of cases. Most would not require any intervention. Sadly a few are incompatible with life. The latter scenario - which is mercifully rare - sets into motion a course of information, counselling and, in most cases, pregnancy termination.

Occasionally the suspected diagnosis will require a confirmatory test, involving amniocentesis (the sampling of the amniotic fluid) or taking a placental or blood sample.

This will entail an inevitable delay of some days before the results are obtained to allow for decision on the definitive action.

There have been cases where, even in the presence of incontrovertible evidence as to the hopelessness of the situation, mothers have declined to have their pregnancies terminated. Full support in the antenatal care is still given in such cases, and appropriate counselling is given regarding the expected delivery of a baby that will not live.

What kind of abnormalities will inevitably lead to recommendation of pregnancy termination?

Conditions which are incompatible with life outside the womb, such as absence of the brain or kidneys.

There are several other rarer conditions. Other conditions such as Edward's syndrome (a chromosomal disorder) may be suspected but will require a confirmatory test. In a confirmed case of Edward's syndrome, termination will be advised because such babies survive for days or at most, a few weeks only.

What conditions which are serious but not necessarily lethal could be missed at twenty weeks?

Occasionally, a diaphragmatic hernia affecting the fetus may be missed. Conditions such as fetal ovarian cysts may not have developed at this stage and will therefore be missed by default. These conditions are rare.

Can a twenty-week scan fail to establish the sex of the fetus?

Most definitely yes.

Sometimes the fetal position does not allow for clear visualisation of the fetal genitalia. This means one cannot say with certainty what sex the fetus is. This is, however, uncommon.

Is every unexplained abnormal finding ominous?

Certainly not. Commonly, shadows that are unexplained are clearly seen, especially in the abdomen. The baby is usually born healthy with no problems whatsoever. The assumption is that this may result from a self-limited viral infection. In the absence of any compelling reasons, such findings only call for observation of the progress of the pregnancy and nothing invasive.

Can spina bifida be missed at twenty weeks of gestation?

This is extremely unlikely. If for any reason the spine is not clearly seen at this stage, a repeat scan will be recommended, usually within four weeks. The resolution is of such high quality that a report of a normal spine means exactly that. This is discussed in detail in Section 18, "Spina bifida and related problems".

Can one tell what colour the baby's hair is likely to be?

No. At this gestation, no hair or even nails can be seen; the image is only in grey-scale, giving the picture in black and white. Having said that; not even a 3D or 4D scan can give you that kind of information.

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Twins in  a first trimester scan.. A scan failing to identify twins in the second trimester is almost impossible.

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