Decoding your dating scan
A dating scan in pregnancy is just one of the many major milestones for you and your baby. It's done to work out the gestational age of your baby, or how far along in pregnancy you are. It tends to be done between 11 and 14 weeks of pregnancy. It helps your doctor or midwife get a better idea of your due date, see if you're having multiples, and check on their growth and development.
During a dating scan, a trained healthcare professional will take a look at your growing baby. The scan can also detect your baby's heartbeat.
When will I have my dating scan?
The dating scan can be done at or shortly after seven weeks of pregnancy, but it often happens a little later. It's the best way to predict your due date (Van den Hof et al 2019). It's also a chance for you to see your baby for the first time and hear their little heartbeat.
When do you get ultrasounds when pregnant?
If you opt to have one, it will happen between 11 weeks and 14 weeks of your pregnancy, along with your prenatal screening tests (SOGC 2019, Van den Hof 2019).
You'll only have a scan before this, between six weeks and 10 weeks, if you've had bleeding, pain, or problems with a past pregnancy (Van den Hof et al 2019). In a few cases it may also be done to confirm a pregnancy.
You'll also be able to have a scan before 11 weeks if you've had miscarriages or an ectopic pregnancy (Van den Hof et al 2019) before.
Some parents have 7 week 3D ultrasounds to get a closer look at their baby, but this is not for medical reasons, and it is not recommended by most medical professionals (SOGC nd(a)).
How accurate are dating scans?
Your baby is measured during your dating scan. This tells your healthcare provider how far along in your pregnancy you are and is a much better method than basing your due date on when you last had a period (SOGC 2019, Van den Hof et al 2019). So, if the scan date differs from your period dates, the scan date will be the one that's used.
Pregnancy weeks are counted from the first day of your last menstrual period (LMP). But that will not tell you when you conceived. Conception usually occurs around 14 days after your LMP, which is when you ovulate (Mongelli 2016). If your cycle is irregular, or you've recently been on the pill, working out your due date from your LMP might not work, which is why a dating scan is a better tool (Van den Hof 2019).
What else will the dating scan reveal?
The scan can check that your baby's heart is beating and that they're developing as they should be (Audibert et al 2017, Van Hof et al 2019).
The dating ultrasound is not designed to look for more subtle abnormalities. That will happen at around 20 weeks. By then, the ultrasound tech or sonographer will be able to see your baby in more detail, because they'll be bigger and their organs will be more developed.
That said, some major problems can be seen at your dating scan, such as issues with the umbilical cord or with your baby's skull or abdominal wall. In that case, you might need further tests, or a second scan to check the first scan's findings.
The dating scan will also show if you're pregnant with twins, triplets, or even more (SOGC nd(b)).
Finding out about having twins early in pregnancy gives you more time to prepare for the birth, and for your doctor and midwife to plan your care.
What happens at a dating scan?
The dating scan takes about 30 minutes (Healthlink BC 2019, SOGC nd(b)).
Most hospitals or clinics will ask you to arrive with a full bladder. The ultrasound tech will put some warm gel on your belly and move a small hand-held device (transducer) over your skin to get a look at your baby (SOGC nd(b)). It will not hurt, although you may feel a little pressure on your tummy and from your bladder - especially if it's full!
If your uterus is very deep in your pelvis, or if you hold weight around your tummy, you may be offered a transvaginal ultrasound (Healthlink BC 2019), which can get a closer view of your baby. If you can relax and take some deep breaths, it should not cause discomfort, and you will not need a full bladder (SOGC 2019).
When your scan is complete, a report will be sent to your healthcare provider, who will go over the results with you.
How big will my baby be at the dating scan?
Your baby is measured from their head to their bottom. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester (Van Hof et al 2019). 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. That's why, from 13 weeks, or if your baby is longer than 84mm, the circumference of their head becomes the best way to measure your baby (Mongelli 2016).
Can I have prenatal screening at the same time?
Most healthcare providers offer prenatal screening to test for Down syndrome and other conditions at the same time as the dating scan (Audibert et al 2017, Van Hof et al 2019).
The first trimester prenatal screening tends to involve a nuchal translucency (NT) ultrasound plus a blood test.
As long as the timings are right you can have the NT ultrasound and blood test at the same appointment as your dating scan (Audibert et al 2017). But you'll have the dating scan first if your due date is unclear (Chitayat et al 2017). That's because the screening test needs an accurate due date to be reliable (Chitayat et al 2017, BORN Ontario nd).
Hormones in your blood vary by your stage of pregnancy. So, a blood test at what you thought was 13 weeks would not be helpful if you were 16 weeks pregnant. It may lead to you being given a high chance for Down syndrome, when that's not the case (AHS 2022).
Are there disadvantages to having a scan?
Ultrasound scans use sound waves to generate a picture. They are safe for you and your baby when performed by trained professionals. Sometimes, having a scan can cause wasted worry. It may show a very minor problem, or something that may get better on its own (Van den Hof et al 2019).
If you have any concerns, talk to your healthcare provider about your options.
More about pregnancy scans:
- Find out if you can get an early scan before 10 weeks.
- Discover whether a dating scan can tell you if you're having a boy or a girl.
- Test your knowledge with our quiz.
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SOGC. nd(a). Non-medical ultrasound Society of Obstetricians and Gynaecologists of Canada. www.pregnancyinfo.ca/postpartum/postpartum/c-section-recovery/Opens a new window [Accessed July 2023]
SOGC. nd(b). Routine ultrasound Society of Obstetricians and Gynaecologists of Canada. www.pregnancyinfo.ca/postpartum/postpartum/postpartum-health-care-and-6-week-postpartum-visit/Opens a new window [Accessed July 2023]
BORN Ontario. nd. Less than 14 Weeks’ Pregnant Prenatal Screening Ontario. www.prenatalscreeningontario.ca/en/pso/prenatal-screening-options/less-than-14-weeks-pregnant.aspxOpens a new window [accessed Dec 2023]
Van den Hof MC, Smithies M, Nevo O, et al. 2019. No. 375-Clinical Practice Guideline on the Use of First Trimester Ultrasound. Journal of Obstetrics and Gynaecology Canada. 1;41(3):388-95.
