Pregnancy Scans
Dating Scan (First Trimester Ultrasound)
Confirms your due date and checks your baby's heartbeat
What is this scan?
A dating scan is the first ultrasound of your pregnancy, usually performed between 6 and 14 weeks. It uses high-frequency sound waves — not radiation — to create images of your uterus and growing baby. The scan confirms that the pregnancy is developing inside the uterus (not ectopic), checks the baby's heartbeat, and measures the Crown-Rump Length (CRL) to calculate your most accurate due date. It also identifies whether you are carrying twins or multiples.
Why is it done?
The dating scan is done to confirm your Expected Delivery Date (EDD) using the most reliable early measurement — the CRL. Your last menstrual period (LMP) gives an estimate, but CRL-based dating is more accurate, especially if cycles are irregular. The scan also confirms a viable pregnancy by detecting a heartbeat, screens for major structural concerns at this early stage, checks the uterus and ovaries, and identifies multiple pregnancies early so appropriate care can be planned. The classical formula is Naegele's rule: LMP + 280 days = EDD. But if CRL-based dating differs by more than 5–7 days, the scan date is used.
What to expect
The scan is usually done transabdominally — a gel is applied to your lower abdomen and a probe is moved gently over the skin. If you are very early in pregnancy (before 8 weeks), a transvaginal scan (an internal probe gently placed in the vagina) may give clearer images. Both methods are safe and painless, though the transvaginal probe may feel slightly uncomfortable. The scan typically takes 20–30 minutes. You may be asked to come with a full bladder for the abdominal scan, as this helps visualise the uterus. No fasting is required. Ultrasound uses sound waves, not X-rays, so it is completely safe for you and your baby.
Normal findings
- ✓ CRL (Crown-Rump Length) measuring within the expected range for your gestational dates
- ✓ Fetal heartbeat detected: 150–180 beats per minute (bpm) is normal in the first trimester
- ✓ Single intrauterine pregnancy, or confirmed multiples with individual heartbeats
- ✓ Gestational sac and yolk sac visible and appropriately sized
- ✓ No abnormalities of the uterus or ovaries noted
When to be concerned
- No heartbeat seen after 7 weeks from your LMP — this needs careful follow-up
- Empty gestational sac (no embryo visible when expected) — possible blighted ovum
- CRL significantly smaller than expected for your dates — may indicate slow growth
- Pregnancy located outside the uterus (ectopic pregnancy) — a medical emergency
- Subchorionic haematoma (blood clot near the placenta) — requires monitoring
Frequently asked questions
Do I need a full bladder for the dating scan?
For a transabdominal dating scan, yes — a moderately full bladder helps the sonographer see the uterus more clearly. Drink about 500–600ml of water an hour before and avoid urinating. If a transvaginal scan is needed, you will be asked to empty your bladder first.
Is the ultrasound safe for my baby?
Yes, completely. Ultrasound uses sound waves, not radiation. Decades of research have found no harmful effects from diagnostic ultrasound on the baby or mother. It is the same technology used in every hospital worldwide.
How accurate is the due date from the dating scan?
A dating scan in the first trimester is the most accurate way to estimate your EDD — accurate to within 3–5 days when done between 8 and 13 weeks. After 20 weeks, scans become less accurate for dating and should not be used to change an already established due date.
What if no heartbeat is seen on the first scan?
If you are fewer than 6–7 weeks pregnant, a heartbeat may not yet be visible — this is normal. Your doctor will ask you to return in 1–2 weeks for a repeat scan. If no heartbeat is seen after 7 weeks (from LMP) with a CRL ≥ 7mm, further evaluation is needed.
Can a dating scan detect twins?
Yes. A dating scan is often the first time a twin or multiple pregnancy is discovered. The type of twinning (identical or fraternal, how many placentas) is also assessed, which matters greatly for how closely the pregnancy will be monitored.
Is the dating scan accurate if I don't know my LMP?
Yes, the dating scan is actually most useful when the Last Menstrual Period (LMP) is unknown or irregular. The sonographer measures the baby's Crown-Rump Length (CRL), which is the most reliable way to determine the age of the pregnancy and the Estimated Delivery Date (EDD) during the first trimester.