The purpose of this examination is to assess:

• Position of pregnancy (ie inside the uterus)
• Number of babies
• Presence of heart motion
• Age of pregnancy (from the first day of menstrual period)

The ultrasound appearance will depend on the age of the pregnancy. At 5 weeks gestation only a gestation sack will be seen. After this a yolk sac will be visible, then a small fetus.

The size of the fetus has to be over 4mm to see a heart beat. A viable fetus can be seen before the heart has begun to beat.

Dating the pregnancy by measuring from the head to the bottom (crown-rump length) is very accurate (+ /- 3 days).

 

An assessment of the baby's structure is difficult before 12 weeks, when a Nuchal Translucency scan can be performed.
Sometimes more than one baby will be seen (twins or triplets) or the pregnancy may not be viable (miscarriage).

Most miscarriages are due to chromosomal abnormalities. Occasionally a pregnancy may occur outside of the uterus, which is know as an ectopic pregnancy.