First Trimester / Dating Ultrasound
What is a dating scan?
This scan is performed in early pregnancy to determine the age of the pregnancy (also known as the gestational age) using measurements. The measurements are from one end of the fetus to the other, known as the crown-rump length, or CRL.
When is it performed?
This scan is usually done if you are unsure about the dates of your last menstrual period (LMP) or have irregular periods. If you have been breast feeding or have recently stopped the oral contraceptive pill you may have had an unusual cycle so you may not be sure of your LMP.
This scan may also be performed also if you have an early pregnancy and have bleeding or pain (even if you have already had a scan or are sure of your last menstrual period) to assess if the pregnancy progressing normally.
How accurate is the pregnancy dating?
A scan performed between 6-11 weeks pregnancy/gestation is accurate to within 3 days.
What else does it show?
- The number of babies
- The presence of the baby’s heart beat
- The position of the pregnancy (check it is in the uterus and not outside, known as an ectopic pregnancy)
- The appearance of the uterus and ovaries
How is it done?
Most of the images can be obtained by trans abdominal ultrasound (this involves scanning through your lower abdomen) using a probe and gel on your skin. This requires a full bladder. Sometimes if the pregnancy is early or better images of your uterus or ovaries is required a transvaginal ultrasound scan is required. This involves an internal probe being gently placed into the vagina with a sterile protective sheath (usually less discomfort than a PAP smear) This allows better images as the probe lies closer to these structures.
Are there some occasions where I will have to return for a repeat early pregnancy scan?
This depends how far along your pregnancy is. Even if you have a positive pregnancy blood or urine test it may be too early to see signs of a pregnancy on ultrasound. Before 5.5 weeks a small fluid sac may be seen in the uterus without sign of a fetus or a fetal heart beat. In these situations a repeat scan one to two weeks later may be necessary to confirm a viable pregnancy and provide accurate dating. Your doctor will also usually correlate the results with blood tests (serum BHCG)
By 6-7 weeks the fetus is usually seen clearly (often requires transvaginal scanning) and its heart beat can be visualised.