An ultrasound scan produces an image of the internal organs of the body on a screen, using the echoes of pulsed sound. Pregnancy scans can establish how many babies you're carrying, check the baby's growth and development, and estimate your due date. However, they can pick up variations in growth that are not real problems but which may cause additional anxiety. Also, they don’t necessarily examine the whole baby in detail and some aspects of development can be missed. Their accuracy depends on the skill of the sonographer, the quality of the machine and the length of time you are scanned.
Scan technicians estimate the length of the pregnancy by measuring the embryo and comparing these results with a chart of standard sizes/gestations. The earlier the scan is performed, the more accurate these comparisons. Even so, scan 'dates' are only accurate to within five days. A scan performed later than 20-24 weeks cannot accurately assess gestation.
A scan before 18 weeks might show:
• How many weeks pregnant you are;
• How many babies there are;
• Whether your baby's heart is beating;
• Any major problems concerning the way your baby's body and limbs have developed.
Around 18-22 weeks, a scan might be offered to check your baby’s development to the spine, arms legs, hands and feet, the heart and other internal organs will be checked.
Scans later in pregnancy will only be offered if there is a particular need for further investigation – see your antenatal care/antenatal testing.
Most reports on the safety of ultrasound have concluded that there is no evidence of adverse effects but advise caution and the need for further research (Midwives Information and Research Service, July 1999).
Do I particularly want to see my baby before he or she is due?
Am I unsure about when my baby is born?
Would I want to know if there was something seriously wrong with my baby?
If you answer ‘yes’ to any of these questions, then having a scan may be the best choice for you. If you answer ‘no’, then you may choose not to have a scan. An extensive article on ultrasound is on the AIMS.