The two most-asked questions in early pregnancy are about timing. When can an ultrasound see a heartbeat? And, a little later, when can it tell the sex? Right behind them: how accurate is the due date, why did the scan show “nothing yet,” and what exactly happens at the big anatomy scan?
The answers are mostly about weeks and millimeters, and knowing them in advance takes a lot of the anxiety out of the first trimester. This guide walks through what ultrasound can see week by week, how dating works, when sex can be determined reliably, what each standard scan is for, and why a scan that is “too early” is not a bad sign.
Counting weeks: the thing that causes most of the confusion
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. Ovulation and conception typically happen about two weeks after that date, so at “6 weeks pregnant” the embryo is only about four weeks old. Every milestone below uses LMP-based weeks, which is what your provider and the ultrasound report use. If your cycles are long, irregular, or you conceived through fertility treatment, the calendar can be off by days or weeks, and that alone explains a large share of “the scan showed less than expected” moments.
What a transvaginal ultrasound sees, week by week
In the first trimester, the scan is usually done transvaginally, with a slim probe placed in the vagina, because it gets much closer to the uterus than a probe on the abdomen and can see structures a week or two earlier. Here is the usual sequence:
| Weeks (from LMP) | What can typically be seen |
|---|---|
| About 4.5 to 5 weeks | A gestational sac: a small fluid-filled circle in the uterus. This confirms the pregnancy is in the uterus, but nothing else yet. |
| About 5 to 5.5 weeks | The yolk sac: a tiny ring inside the gestational sac. Its presence confirms a true pregnancy sac. |
| About 6 weeks | The embryo itself, a few millimeters long, and usually a flickering heartbeat. At this stage the heart rate is often around 100 to 120 beats per minute. |
| 7 to 8 weeks | A clear embryo with a strong heartbeat, often 140 to 170 beats per minute. Crown-rump length (CRL) is measured for dating. |
| 9 to 10 weeks | Head, body and limb buds are distinguishable; movement may be seen. |
| 11 to 14 weeks | The fetus looks like a baby. This is the window for the nuchal translucency scan (see below). |
The key point: before about 6 weeks, not seeing a heartbeat is expected, not concerning. A scan at 5 weeks that shows only a sac is simply early. Your provider will usually repeat it in one to two weeks, by which time the embryo and heartbeat have had time to appear.
“Viability”: what the scan is checking and how doctors avoid false alarms
A first-trimester “dating and viability” scan answers three questions: Is the pregnancy in the uterus (not ectopic)? Is there an embryo with a heartbeat? How far along is it? Because an early scan can look empty simply from being early, radiologists use deliberately strict criteria before concluding that a pregnancy is not progressing. The criteria adopted by the Society of Radiologists in Ultrasound are:
- an embryo measuring 7 mm or more with no heartbeat, or
- a gestational sac measuring 25 mm or more with no embryo inside, or
- no embryo with a heartbeat on a follow-up scan done a set number of days after an earlier scan (about two weeks after a sac was seen without a yolk sac, or 11 days or more after a sac with a yolk sac).
Anything short of those thresholds, such as an embryo under 7 mm without a visible heartbeat, or a sac of 16 to 24 mm with no embryo, is classed as “concerning but not diagnostic,” and the standard response is a repeat scan in 7 to 14 days rather than any decision. If you have been told to come back in a week or two, that is this rule in action, and it protects viable pregnancies from being misjudged.
How accurate is the due date from an ultrasound?
More accurate than the calendar, especially early. In the first trimester the embryo is measured from crown to rump (CRL), and because early embryos grow at an extremely consistent rate, this measurement predicts the due date to within about five to seven days. It is the most accurate dating available, which is why an early scan date usually takes precedence over the LMP date if the two disagree by more than a few days. Dating by ultrasound gets less precise as pregnancy advances: roughly plus or minus a week in the early second trimester, and two to three weeks by the third, because babies of the same age vary more in size as they grow. For that reason, a due date set in the first trimester is normally not changed by later scans.
When can ultrasound tell the sex?
This depends on gestational age far more than on the skill of the sonographer or the quality of the machine.
- Before about 12 weeks: not reliably. The external genitals look the same in both sexes until around 11 to 12 weeks, and studies that tried to call sex with an embryo under 50 mm CRL (before about 11.5 weeks) were right only about a third of the time.
- 12 to 14 weeks: increasingly possible. By a CRL of 60 mm (around 12.5 weeks) experienced sonographers using the angle of the genital tubercle were close to 100 percent accurate in one study. In everyday practice many clinics still say “probable” at this stage, because position and image quality vary.
- 14 to 16 weeks: usually clear when the baby cooperates.
- 18 to 22 weeks (the anatomy scan): the standard time to confirm. Occasionally a stubborn position (legs crossed, umbilical cord in the way) means a second look is needed.
If you want to know earlier, cell-free DNA blood testing (often called NIPT), available from around 10 weeks, reports fetal sex alongside screening for chromosomal conditions. Ask your prenatal provider.
The standard scans, and what each one is for
| Scan | Usual timing | Purpose |
|---|---|---|
| Dating and viability | 6 to 13 weeks (often 7 to 9) | Confirms location in the uterus, heartbeat, number of embryos, and sets the due date from CRL. Usually transvaginal. |
| Nuchal translucency (NT) | 11 weeks to 13 weeks 6 days (CRL 45 to 84 mm) | Measures the fluid at the back of the baby’s neck as part of first-trimester screening for chromosomal conditions, usually combined with blood tests. Ordered by your prenatal provider. |
| Anatomy scan (level 2) | 18 to 22 weeks | A detailed survey of the brain, face, spine, heart, abdomen, kidneys, limbs, placenta and amniotic fluid. Sex is confirmed if you want to know. The most important scan of the pregnancy. |
| Growth scan | Third trimester, as needed | Measures the baby (estimated fetal weight), checks position, fluid and placenta. Done when there is a specific reason, such as size concerns, diabetes or high blood pressure. |
The anatomy scan is part of your prenatal care pathway, so at UltraVision Imaging it requires a referral from your obstetric provider and the report goes to them directly. The first-trimester dating scan and the limited growth scan can be booked directly.
About “keepsake” and elective scans
A non-diagnostic heartbeat or gender visit is a lovely thing to do, and we offer one, but it is important to be clear about what it is: a relaxed session to hear the heartbeat and, when it is visible, see the sex. No measurements are taken for medical purposes, no report is written, and nothing is sent to your doctor. It does not replace any of the scans above, and you must be under the care of a prenatal provider. If anything looks concerning during an elective visit, our sonographer will tell you and recommend you follow up with your provider promptly.
Is ultrasound safe in pregnancy?
Diagnostic ultrasound has been used in pregnancy for decades and has no known harmful effects at the energy levels used for medical imaging. It does not use radiation. Professional bodies do recommend that scans be done for a medical reason, by trained operators, keeping exposure time as low as practical, which is why elective scans should be short and should not be repeated for entertainment.
Practical details
- Preparation: for a first-trimester scan, arrive with a full bladder (about 32 ounces of water an hour before). It helps the transabdominal part of the exam; you will be asked to empty it before any transvaginal portion. Later scans need no preparation. Our preparation guide has more detail.
- Bring: your LMP date or your fertility clinic’s dating, and any prior scan reports.
- Results: the sonographer cannot give a diagnosis during the scan (here is why), but a board-certified radiologist’s written report is typically ready within 24 hours and can go straight to your prenatal provider.
- Bleeding or severe pain in early pregnancy should be evaluated urgently by your provider or an emergency room rather than booked as a routine scan.
To book a first-trimester dating and viability scan, or a third-trimester growth scan, at our private clinic or at home, enter your ZIP to see times or call (916) 751-1944. If you have a referral for an anatomy scan, book online and we will confirm once it is on file.
Frequently asked questions
When can an ultrasound detect a heartbeat?
Usually around 6 weeks from the first day of your last period on a transvaginal scan, when the embryo is only a few millimeters long. Before 6 weeks, seeing a sac without a heartbeat is expected, and the scan is repeated in one to two weeks.
Is it bad if no heartbeat was seen at 5 or 6 weeks?
Not by itself. Radiologists only diagnose a non-progressing pregnancy when strict thresholds are met, such as an embryo of 7 mm or more with no heartbeat or a sac of 25 mm or more with no embryo. Anything short of that is “too early to say,” and a follow-up scan in 7 to 14 days is the standard next step.
How accurate is ultrasound for the due date?
A first-trimester scan dates the pregnancy to within about five to seven days using the crown-rump length, which is more accurate than the last-period date. Accuracy falls to about a week in the early second trimester and two to three weeks in the third, so a due date set early is normally kept.
When can ultrasound tell the sex of the baby?
Not reliably before about 12 weeks. Accuracy becomes high from around 12.5 weeks (crown-rump length 60 mm or more) in experienced hands, is usually clear by 14 to 16 weeks, and is confirmed at the 18-to-22-week anatomy scan. A cell-free DNA blood test from about 10 weeks can also report sex.
What is the anatomy scan and when is it done?
The anatomy scan, done between 18 and 22 weeks, is a detailed survey of the baby’s brain, face, spine, heart, abdomen, kidneys and limbs, plus the placenta and amniotic fluid. It is part of prenatal care and is ordered through your obstetric provider; at UltraVision Imaging it requires a referral.
Why is the first-trimester scan transvaginal?
The transvaginal probe sits much closer to the uterus than a probe on the abdomen, so it can see the sac, yolk sac and heartbeat one to two weeks earlier and in more detail. It is brief and generally causes only mild pressure.
Is ultrasound safe for the baby?
Diagnostic ultrasound uses sound waves, not radiation, and has no known harmful effects at medical imaging levels. Scans should be done for a medical reason by trained operators and kept as short as practical.
Can I book a pregnancy ultrasound without a referral?
At UltraVision Imaging, the first-trimester dating and viability scan and the limited growth scan can be booked directly. The second-trimester anatomy scan requires a referral from your prenatal provider.
This article is general health information, not medical advice. It does not replace a conversation with your own physician, who knows your history and can interpret results in context. If you have symptoms that worry you, contact your doctor; if you think you are having a medical emergency, call 911.