A dating scan is an ultrasound scan done between 8 and 14 weeks of pregnancy to help estimate your baby’s due date. In early pregnancy, most babies of the same gestational age are about the same size. A dating scan measures your baby and this helps your doctor estimate how long you have been pregnant, and when your baby is due. Not everyone needs a dating scan, but they can be very helpful if you aren’t sure when you conceived.
It’s important to have an accurate estimated due date (or EDD) for your baby so you can have the recommended tests at the right time. Knowing how far along you are is also important if your baby is born prematurely, or if you haven’t given birth by your estimated due date and you’re thinking about having your labour induced.
When is a dating scan usually done?
A dating scan is done between 8 and 10 weeks. At 6 weeks gestation your pregnancy test will be positive. A transvaginal ultrasound examination may show a tiny sac in the lining of the womb about 6mm in size.
This is going to be your first peek at your growing bundle of joy! Don’t expect to see a lot of definition or details this early in the game.
For now, you’ll see a little figure that looks something like an oblong bean. If there are twins, you might see two figures. The head is still nearly the same size as the rest of the body.
You’ll also see the gestational sac, the fluid-filled space around your baby(s). Within it, you can also see the yolk sac, which is a bubble-like structure. Depending on the location, you might even get to hear their heartbeat, too.
The main reasons for the 8-week ultrasound may be to confirm a pregnancy, determine a due date, and confirm the baby’s heartbeat. First, your doctor or technician will look for key physical indicators, like a gestational sac and a fetal pole, to verify the pregnancy is in the uterus. This is may be your first indication of twins.
A little note from me here – I highly recommend waiting until at least 8 weeks for this scan as every day makes a difference. Getting it too early is exciting but the yolc sac and fetal pole may not be visible yet which will only cause you to stress. Although the ultrasonographer will want to reassure you, she also does not know as it is a matter of time, so they only recommendation will be to return again in 7 – 10 days for a repeat scan. However, imagine how much you will worry in these 7 – 10 days which is not good for your developing baby. Rather use this time to connect with your body and your growing baby and tune into your inner knowing.
Once they’ve confirmed that you’re pregnant, the next step is to verify your projected due date. Even though you might have initially received a projected due date at an earlier appointment, it’s not always accurate. The initial due date is determined by confirming the first day of your last period, deducting 3 months, and then adding 1 year and 7 days. But because not every person’s menstrual cycle is the same length of time, these projections can be off.
With an ultrasound, a physician or technician can determine gestational age and due date by measuring the size of your fetus. The accepted method for early pregnancy dating is the crown-rump length (CRL) measurement because it’s the most accurate (within about 5 to 7 days) in the first trimester.
Who needs a dating scan?
Most babies are born about 38 weeks after conception. Since many women ovulate (release an egg that may then be fertilised) and conceive about 2 weeks after their last period, this is often about 40 weeks since the beginning of their last period. That’s why people often talk about pregnancy lasting for 40 weeks.
Women with a regular 28-day cycle can calculate an estimated due date for their baby by counting 40 weeks from the first day of their last menstrual period. This may not be so simple or accurate in other situations, like if you have long or irregular cycles, don’t remember when you had your last period, or if you got pregnant while taking contraception that affected your cycle.
When you can’t see the baby or a heartbeat
Sometimes you can’t see the fetus or hear a heartbeat — but that doesn’t always mean the worst. Sometimes it means that your calculations on the conception date were off.
If you ovulated and conceived later than you initially assumed, you might be getting an ultrasound too early to get a physical confirmation. In other scenarios, you might have large fibroids or anatomic issues with the uterus that can make screening your uterus more difficult.
But in some situations, it might not be the news you hoped for. Occasionally, the absence of a visible fetus in the uterus could mean you have an ectopic pregnancy, where the embryo implanted outside of the uterine cavity.
Other times, you might have experienced a blighted ovum — when the embryo fails to develop or stops developing, yet a gestational sac remains. Or, unfortunately, you might have miscarried.
What happens during a dating scan?
In early pregnancy, ultrasounds including dating scans can be done through your abdomen (tummy) or vagina. The method used will depend on a few factors, including how far along your pregnancy is and your body shape.
If your scan is being done along your abdomen (known as a ‘transabdominal ultrasound’), you will be asked to drink a few cups of water before you arrive so your bladder is full. This can make it easier to see inside your uterus (womb). The sonographer will apply some gel and gently move the ultrasound probe along your abdomen. It doesn’t usually hurt.
If your scan is done through your vagina (known as a ‘transvaginal ultrasound’), a small ultrasound probe is lubricated and gently inserted into your vagina. The probe may be a little uncomfortable but usually isn’t painful. Scans done this way can give more detailed pictures because the probe is closer to your uterus.
Ultrasounds, including dating scans, do not harm you or your baby or increase your risk of miscarriage.
What do the results mean?
During the scan, the sonographer will measure your baby’s length from head to bottom, known as their ‘crown-rump length’ (CRL). This measurement can help estimate your baby’s gestational age and when it is likely to be born.
Having an accurate estimated due date is helpful, but it’s also important to remember it’s only an estimate. Most babies are not born on their due date.
During the test, the sonographer may also:
- confirm that your pregnancy is in the right place and is not ectopic
- look for your baby’s heartbeat
- check if you are carrying more than one baby
- check that your baby’s body organs are developing normally