Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used.

From 6 to 11 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.

Accuracy of CRL after 12 weeks in predicting gestational age diminishes and is replaced by measurement of the width of the fetal head (biparietal diameter or BPD).

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During the examination, the fetus is seen by abdominal ultrasound.

Occasionally the view is not clear and it may be necessary to perform a vaginal scan.

Occasionally the nasal bone cannot be seen at 11 weeks as it is too early in the pregnancy.

If this is the case and this creates some concern then the scan can be repeated a week later.

Crown Rump Length: Gestational age = 6 weeks plus (CRL x days).

For example, a CRL of 23 mm would correspond to a gestational age of 9 weeks and two days (6 weeks plus 23 days = 9 weeks and 2 days).

First trimester ultrasonic scans may show 'soft' markers for chromosomal abnormalities, such as an increased fetal nuchal translucency (back of the neck) to enable detection of Down's syndrome fetuses.

The test is performed between 11 weeks and 13 weeks 6 days into your pregnancy.

If you are unsure of your period dates or have an irregular cycle, we suggest that you contact your doctor, midwife or health care provider as early as possible to arrange for a dating scan.

There is no hard and fast rule for the number of scans you should have during pregnancy.

You will probably be give you a printout of your baby by the sonographer as a keepsake, it is important to remember the main purpose of the scan.