When a pregnancy shows on ultrasound: sac, embryo, heartbeat
A positive test and an ultrasound that shows nothing yet is the most common reason for worry in early pregnancy. Below are the weeks at which each thing becomes visible, and what to do if the scan came too early.
How the weeks are counted
Pregnancy weeks are counted from the first day of the last period, not from conception. About two weeks separate the two. So at five weeks of pregnancy the embryo itself is only around three weeks old.
This is where most of the confusion starts. With a cycle longer than 28 days, or an irregular one, ovulation happens later, the pregnancy is genuinely younger than the period date suggests, and a scan booked "on time" turns out to be early. A week either way changes everything at this stage: it is the difference between an empty sac and a visible heartbeat.
What shows when, week by week
These timings are for a transvaginal scan. Through the abdomen everything appears roughly a week later.
| Week | What is visible | Notes |
|---|---|---|
| 4+3 – 5+0 | Gestational sac, 2–3 mm | Usually once hCG is above 1500–2000 IU/l |
| 5+0 – 5+3 | Yolk sac inside the gestational sac | The first reliable proof the pregnancy is inside the uterus |
| 5+5 – 6+2 | Embryo 2–5 mm, in most cases already with a heartbeat | The heartbeat is often seen before the embryo can be measured |
| 6+3 – 7+0 | A heartbeat in virtually every ongoing pregnancy | No heartbeat at a crown-rump length of 7 mm or more means the pregnancy has failed |
| 11+0 – 13+6 | First-trimester screening, nuchal translucency | Done at a crown-rump length of 45–84 mm |
The two thresholds in the last column are deliberately cautious. A gestational sac of 25 mm with no embryo, and an embryo of 7 mm with no heartbeat, are the points at which a failed pregnancy can be diagnosed with confidence. Anything below those numbers simply calls for a repeat scan — the diagnosis is not made early.
Transvaginal or abdominal
Until about twelve weeks the uterus sits behind the pubic bone, and a vaginal probe is a few centimetres closer to it. At this stage the difference in resolution decides everything: what is a grey smudge through the abdomen is a measurable sac transvaginally.
A transvaginal scan in pregnancy is safe. It does not cause miscarriage or bleeding, and it needs an empty bladder rather than a full one. An abdominal scan is the opposite: drink about half a litre of water an hour before the appointment.
A positive test and an empty uterus
This is not one scenario but three, and a single scan usually cannot tell them apart:
- Too early. The most likely explanation, especially with irregular cycles. A repeat scan in 7–10 days settles it.
- Pregnancy of unknown location. A positive test, an empty uterus, and nothing visible outside it either. hCG is then followed every 48 hours: in a normal early pregnancy it rises by at least half.
- Ectopic pregnancy. This needs prompt investigation, because the risk is a ruptured tube. There is usually a mass beside the uterus or free fluid in the pouch of Douglas.
So with a positive test and an empty uterus the standard is not open-ended waiting but a booked follow-up: a blood test, then a repeat scan.
When to have the first scan
If nothing is worrying you, 6–8 weeks is the most useful window. The embryo and the heartbeat are visible by then, the pregnancy can be confirmed as intrauterine, the number of embryos counted, and the due date set — at this stage to within a few days.
An earlier scan makes sense if there are symptoms, after a previous ectopic pregnancy or miscarriage, after ovulation induction or IVF, or if an IUD is in place.
The next scan is first-trimester screening, between 11+0 and 13+6 weeks.
Short answers
When is the embryo visible on ultrasound?
Usually between 5+5 and 6+2 weeks on a transvaginal scan, at a length of 2–5 mm. Through the abdomen, roughly a week later.
When is the heartbeat visible on ultrasound?
In most pregnancies from about 6 weeks. From 6+3 – 7+0 weeks a heartbeat is seen in virtually every ongoing pregnancy. Its absence at an embryo length of 7 mm or more means the pregnancy has failed.
When does an ectopic pregnancy show on ultrasound?
Usually from 5–6 weeks: an empty uterus with hCG above the threshold, a mass beside the uterus, sometimes free fluid. The diagnosis rests on ultrasound together with repeated hCG measurements, not on a single scan.
Is a transvaginal scan safe in pregnancy?
Yes. It does not cause miscarriage or bleeding. Until twelve weeks it gives a markedly clearer picture than an abdominal scan.
This text describes the general picture and does not replace a consultation. Timings and indications differ from woman to woman and are decided at the appointment, from your history and test results.