In most cases, this will be combined with a blood test for increased accuracy.
In your second trimester you'll be offered an anomaly scan between 18 weeks and 21 weeks.
But if your baby's lying in an awkward position, it's not always easy to tell.
Plus, some hospitals have a policy of not revealing the gender of a baby.
A full bladder helps the ultrasound echoes to reach your womb, giving the sonographer a good view of your baby.
If your baby's still deep in your pelvis, or if you're overweight, the image won't be very clear.
Hard tissues, such as bone, reflect the most sound waves and so make the biggest echoes.
If you consent, you may also be scanned by someone who is training in ultrasound, under the direct supervision of a qualified sonographer.
Having a scan with a trainee can be an interesting experience as it can let you know exactly what the sonographer is looking for during your scan. The investigation and management of the small-for-gestational-age fetus.
Your first scan can be very exciting because it gives you a first glimpse of your baby.
Your sonographer may even print out the image of your baby and give it to you as a keepsake.