First Trimester Ultrasound: Weeks, What to Expect, and How to Prepare
First Trimester Ultrasound: Ideal Weeks, What It Evaluates, and How to Prepare
A complete guide for expectant mothers: when to have your first pregnancy ultrasound, what the doctor measures (CRL, nuchal translucency, heartbeat), what each piece of information means, and how to prepare properly.
Reviewed by diagnostic imaging specialists · GSMED Imaging ColombiaThe first trimester ultrasound is the first visual encounter between expectant parents and their baby. But beyond the emotional moment, this study has enormous diagnostic value: it confirms the pregnancy, estimates gestational age with the greatest possible precision, evaluates the vitality of the embryo or fetus, and—when performed between weeks 11 and 13+6—allows for the estimation of the risk of certain chromosomal abnormalities by measuring the nuchal translucency (NT).
In Colombia, the Ministry of Health's prenatal care protocols include the first trimester ultrasound as part of routine obstetric check-ups. Knowing what it evaluates, when to have it, and what the results mean allows pregnant women to arrive better prepared and make the most of their time with their doctor or sonographer.
What weeks is the first trimester ultrasound performed?
There are actually two ultrasound moments in the first trimester, with different objectives:
Weeks 11–13+6
This is the first trimester ultrasound itself. It provides the most diagnostic information and should be performed when the CRL (crown-rump length of the fetus) is between 45 and 84 mm, which corresponds exactly to weeks 11+0 and 13+6 days.
- Nuchal translucency (NT) measurement
- Fetal anatomical evaluation (early morphology)
- Precise CRL measurement to date the pregnancy
- Nasal bone, ductus venosus, tricuspid regurgitation
- Maternal uterine and ovarian evaluation
✅ Which is the most important?
The ultrasound of weeks 11–13+6 is the one that offers the most information and the only one that can evaluate nuchal translucency. If there was no bleeding or early complications, the first ultrasound can be done directly in this window without the need for a previous ultrasound at 6–8 weeks.
Preparation for the first trimester ultrasound
Preparation depends on the route through which the study will be performed:
🔵 Transabdominal route (weeks 11–14)
For this route, a moderately full bladder is needed. The usual recommendation is to drink 4–6 glasses of water (approximately 500–800 mL) between 30 and 60 minutes before the study and not urinate. The full bladder acts as an acoustic window and elevates the uterus, facilitating visualization.
🟠 Transvaginal route (weeks 6–10)
For the transvaginal ultrasound, the preparation is the opposite: the bladder must be empty. A full bladder would push the uterus backward, making visualization difficult. It is recommended to urinate before entering the ultrasound room.
💡 Practical tip: Bring your prenatal card or any previous lab results (beta-hCG, previous ultrasounds). It is also useful to know the exact date of your last menstrual period (LMP), although the doctor will calculate the gestational age from the CRL measured in the study.
What does the first trimester ultrasound evaluate?
A complete first trimester ultrasound (weeks 11–13+6) evaluates both the fetus and the mother:
Fetal evaluation
Confirmation of the fetal heartbeat. Normal heart rate 120–160 bpm.
Crown-rump length to calculate exact gestational age and estimated due date.
Single or multiple pregnancy. In twins, determines chorionicity and amnionicity.
Fluid in the back of the fetus's neck. Central parameter of the first trimester screening.
Absent in ~60–70% of fetuses with trisomy 21. Additional risk marker.
Evaluation of the head, brain (rhombencephalon), spine, chest, abdomen, and limbs.
Maternal evaluation
- Location of the pregnancy (intrauterine / ectopic)
- Uterine fibroids and their relationship with the placenta
- Ovaries: ovarian cysts (corpus luteum, endometriomas, dermoids)
- Cervical length (risk marker for premature birth)
Key measurements: CRL and nuchal translucency
📏 CRL (Crown-Rump Length — Crown-Rump Length)
CRL is the most accurate measurement available for calculating gestational age, with a margin of error of only ±5–7 days. It is measured with the fetus in a neutral position (neither flexed nor extended) from the top of the head to the coccyx.
| Weeks + days | Average CRL | Expected finding |
|---|---|---|
| 6 + 0 | 4–5 mm | Yolk sac visible; heartbeat may be present |
| 8 + 0 | 16 mm | Spontaneous embryo movements visible |
| 11 + 0 | 45 mm | Start of the window for nuchal translucency |
| 12 + 0 | 58–62 mm | Ideal week for first trimester ultrasound |
| 13 + 6 | 84 mm | End of the window for nuchal translucency |
🔍 Nuchal translucency (NT)
Nuchal translucency is the accumulation of fluid under the skin at the back of the fetus's neck. It is the most important ultrasound marker in first-trimester screening for chromosomal abnormalities (primarily trisomies 21, 18, and 13) and congenital heart disease.
< 3.0 mm
NORMAL
Low risk. No additional studies are required based on this parameter alone.
3.0–3.5 mm
BORDERLINE
Increased risk. Evaluated together with biochemical markers and other ultrasound markers.
> 3.5 mm
INCREASED
Significantly elevated risk. Genetic counseling and diagnostic testing are recommended.
An abnormal result does not confirm a chromosomal abnormality; it is an indication for further investigation.
First-trimester ultrasound findings and what they mean
This may be normal before 6 weeks. If the sac measures more than 25 mm and no embryo is visible, it may indicate an anembryonic pregnancy (blighted ovum). Clinical correlation and follow-up are always required.
An embryonic heart rate of less than 100 bpm before 7 weeks or less than 120 bpm after 7 weeks is associated with an increased risk of pregnancy loss. Close follow-up is required.
Accumulation of blood between the chorionic membranes and the uterine wall. A frequent cause of bleeding in the first trimester. Small hematomas usually resolve spontaneously; large ones are associated with an increased risk of pregnancy loss.
The absence of cardiac activity in an embryo with a CRL greater than 7 mm is diagnostic of fetal death. In some centers, a second scan (7–10 days later) is required before confirming the diagnosis.
Absence of an intrauterine sac with elevated β-hCG and possible visualization of an extrauterine sac (most commonly in the fallopian tube). Medical emergency. Transvaginal ultrasound is the preferred diagnostic tool.
Does your clinic need high-resolution obstetric ultrasound equipment?
At GSMED Imaging, we distribute ultrasound machines with transducers designed for first-trimester obstetric imaging, providing the resolution needed for accurate nuchal translucency measurements. We advise clinics and gynecologists in Colombia.
Request a free demonstration →Frequently Asked Questions About First-Trimester Ultrasound
Common questions from expectant mothers during the first trimester of pregnancy.
