This ultrasound helps to accurately establish your baby's gestational age. This is particularly useful if you are not sure when your last menstrual period was, if you have irregular periods, or you became pregnant immediately after you stopped using contraception such as the pill. At this stage of pregnancy, your baby can be measured from crown to rump (from the top of the baby's head to its bottom). In addition to establishing your due date and the timing of other screening and diagnostic tests, accurate dating of your pregnancy is important since it helps avoid misdiagnosis of problems such as poor fetal growth. Your due date will usually be changed at this scan if there is more than a five to seven day difference between your menstrual dates and the dates based on the crown-rump length.
A scan may be given before 10 weeks if you have bleeding or pain to rule out the possibility of a miscarriage or ectopic pregnancy.
How the scan is done
During an ultrasound scan, high-frequency sound waves are emitted through the abdomen via a handheld device called a transducer. As the sound waves hit solid tissue, they translate into an image that is viewed on a computer screen and interpreted by the sonographer.
At this scan, you will need to drink plenty of water to raise the uterus and provide a clearer image. The sonographer will put some cold gel on your abdomen to maximize contact with the skin and will then move the transducer gently over the area.
What can be seen on a first scan?
In addition to confirming your dates, your first scan may reveal some other useful information.
- This scan usually confirms whether you have a single or multiple pregnancy (twins, triplets, or more).
- Uterine anomalies can be seen, such as a double uterus, although this is rare. Uterine fibroids (benign tumors) will also be identified.
- The scan may reveal an ovarian cyst (corpus luteum) on the ovary that produced the egg. These are common and can persist in the first trimester.
- Major anomalies may be seen, but most are diagnosed at the 20-week scan when the organs are seen (see What the scan reveals).
By 12 weeks the fetus has taken on a human appearance. The forehead, eye sockets, and small button nose are all visible in profile.
Measuring the diameter of your baby's head (the biparietal diameter) helps assess your baby's growth and date your pregnancy.