Your baby's head, limbs, hands, and feet can be seen, as well as his abdominal wall and some organs, such as his stomach.
The dating scan isn't designed to look for more subtle abnormalities, though.
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Obstetric ultrasound was first introduced in the late 1950s.
It may also help you to avoid feeling disappointed towards the end of your pregnancy if you think your baby is late when he's not.
The scan can check that your baby's heart is beating nicely and that he's developing normally.
The first ultrasound scan is usually performed before 15 weeks.
Pregnancies affected by aneuploidy (abnormal chromosome number) will have sonographic markers.
However, 50-80% of affected cases will already be identified by triple test, maternal age and nuchal translucency measurements.
Indications for a 'marker' scan include: These are set by the Royal College of Obstetricians and Gynaecologists to assure the quality of service provision.
How big your baby will be depends on when you have the scan: After 13 weeks, your baby can curl up and stretch out, so measuring the length becomes less accurate.