What are the benefits of an early pregnancy scan?
Occasionally, an ultrasound scan may have to be performed early in pregnancy, probably within the first twelve weeks. This is usually in situations where the woman wants to confirm that she is pregnant. This may arise for a variety of reasons.
Other indications for an early scan may be:
Confirming the viability of the pregnancy e.g. in instances where there has been vaginal bleeding in early pregnancy.
Identifying the location o f the pregnancy. This is in cases where there is increased possibility of ectopic pregnancy, such as in women with a history of previous ectopic pregnancy, a history of pelvic infection or those suffering localized lower abdominal pain in pregnancy.
Excessive morning sickness, which may be a sign of multiple pregnancy or even an abnormal pregnancy (molar pregnancy). An early scan will establish this.
Sudden disappearance of pregnancy symptoms or unexplained production of milk in early pregnancy. This may be a warning that the pregnancy is no longer viable. A scan should establish whether this is the case.
In assisted conception (IVF). An early scan is important to confirm the viability, the number of implanted embryos and establish whether the pregnancy is ectopic (there is increased risk of this in some forms of assisted conception).
When is a late scan of benefit?
Some pregnancies, by their very nature, are regarded as high risk. Multiple (twins, triplets etc.) pregnancies come immediately to mind. Other pregnancies are apparently normal at twenty weeks but problems occur later in the course of the pregnancy. These will usually be detected through clinical monitoring of the progress of the pregnancy. In some cases, the midwife or doctor may notice that the growth is lagging behind or is excessive.
Also, it may be noted that there is too much or too little fluid around the baby.
In other cases, the mother may be concerned by the rate of activity of the fetus (usually too little). All these and several other factors may be indications for performing a scan in the later phase of pregnancy. In some cases, usually when there is concern at the rate of growth of the fetus, serial scans may be performed to closely and accurately monitor the progress. (See chapter 4, "Ultrasound".)
Different types of scans
How does an abdominal scan differ from a vaginal scan?
A vaginal scan will normally be performed in early pregnancy. This is because it gives quite significantly better views of the womb and its content at this early stage, compared to the abdominal scan. Consequently, the information obtained is better and more detailed.
After about ten weeks, the abdominal route may be just as good, for most mothers. For mothers who are very obese, a vaginal route may continue to be ideal for several weeks more. In late pregnancy, a vaginal route may be used only occasionally, especially when there is difficulty in establishing how close the placenta is to the cervix. Again, this may occur in women who are overweight, where views through the abdominal route are not as good.
Is the vaginal scan safe?
Like the abdominal scan, the vaginal scan has not been shown to have any adverse effect on either the mother or baby. Remember that sound waves are used in this technology. There is no radiation.
Accuracy of ultrasound scans
How accurate is the scan at estimating the gestation?
In the first trimester (the first fourteen weeks), ultrasound scan is very accurate, with a margin of error of only about three or four days either way. However, if a scan is done too early, that is before 7 weeks, it may not be so accurate. The accuracy reduces as the pregnancy advances and may be out by as much as one week at eighteen to twenty weeks.