Articles (EN)

Ultrasound during pregnancy: timing and what to look at

2026-05-27 12:00 Pregnancy
In short: In uncomplicated pregnancies, three routine ultrasound screenings are performed: at 11–14 weeks, 18–21 weeks, and 32–34 weeks. Each study solves its own tasks - from assessing the risk of chromosomal abnormalities to determining the position of the fetus before childbirth. Ultrasound is safe for the mother and fetus at any stage.

Why is ultrasound done during pregnancy?

Ultrasound is the main method of visualizing the fetus and assessing the course of pregnancy. It is safe, non-invasive, and allows you to obtain information that is not available by other means. With the help of ultrasound, the doctor assesses the size and development of the fetus, the work of the heart, the amount of amniotic fluid, the condition of the placenta and cervix.
Ultrasound does not emit ionizing radiation, which fundamentally distinguishes it from X-rays and CT. Long-term studies have not revealed the harmful effect of diagnostic ultrasound on the fetus. Nevertheless, the studies are carried out for medical reasons, and not "just in case" - this is what is included in routine screenings.

First screening: 11-14 weeks

The first ultrasound screening is carried out in a strictly defined time window - from 11 weeks 0 days to 13 weeks 6 days of pregnancy. This is one of the most important studies of the entire pregnancy.
What is assessed:
The coccyx-parietal size (CTE) is the main indicator for clarifying the gestational age.
Nuchal space thickness (NT) is the space between the skin and the soft tissues of the fetal neck. An increase in TVP (more than 3 mm) is a marker of risk for chromosomal abnormalities, primarily Down syndrome.
Nasal bone – its absence at this stage increases the risk of trisomy 21.
Fetal heart rate.
Fetal anatomy – what is available for evaluation at this stage.
Number of fetuses and chorion.
Condition of the uterus, appendages, cervix.
The first screening is combined with a blood test for biochemical markers - PAPP-A and free β-hCG. Together, they make up a combined first-trimester prenatal screening to calculate an individual's risk of chromosomal abnormalities (trisomy 21, 18, 13).

Second screening: 18-21 weeks

The second screening is the most detailed in terms of assessing the anatomy of the fetus. It is at this stage that most of the organs and systems of the fetus are sufficiently formed for the doctor to be able to carefully examine them.
What is assessed:
Fetometry – biparietal and fronto-occipital dimensions of the head, abdominal circumference, femur length. Based on these data, the estimated weight of the fetus and the correspondence of the size to the term are calculated.
Fetal anatomy – brain (ventricles, cerebellum, corpus callosum), face (lip, palate, profile), spine, heart and large vessels, lungs, abdominal organs (stomach, intestines, kidneys, bladder), limbs.
The sex of the fetus is at the request of the parents.
Placenta – localization, structure, degree of maturity.
Amniotic fluid (amniotic index).
Cervix is the length of the cervical canal (transvaginally if necessary).
At the second screening, the doctor can identify most of the structural abnormalities of development that can be diagnosed prenatally. If abnormalities are detected, additional studies are prescribed - echocardiography, consultation with a geneticist, sometimes invasive diagnostics.

Third screening: 32-34 weeks

The third routine screening assesses the condition of the fetus and readiness for childbirth.
What is assessed:
Fetometry and estimated fetal weight.
Position and presentation of the fetus (cephalic, pelvic, transverse).
Condition of the placenta: localization, degree of maturity (according to Grannum), thickness, presence of calcifications.
Amount of amniotic fluid.
Umbilical cord condition and blood flow (Doppler).
Doppler - an assessment of blood flow in the vessels of the fetus, umbilical cord and uterus - is carried out at the third screening without fail. It allows you to detect placental insufficiency at an early stage. Information about fetal presentation is important for planning the mode of delivery.

Additional and unscheduled ultrasound

In addition to the three routine screenings, ultrasound can be performed at any time during pregnancy if medically necessary:
Early ultrasound (up to 11 weeks) - to confirm pregnancy, exclude ectopic, clarify the term in case of an irregular cycle.
In case of bloody discharge or pain - to exclude the threat of miscarriage.
In case of decreased or absent fetal movements.
In case of signs of preeclampsia or other complications.
CTG (cardiotocography) is not an ultrasound, but complements it in the third trimester.

How to prepare for an ultrasound

Transabdominal ultrasound (sensor on the abdomen) – in the first trimester, it is recommended to come with a full bladder (drink 0.5-1 liter of water in 30-40 minutes and not urinate). In the second and third trimesters, the bladder is not needed - the uterus is already quite large.
Transvaginal ultrasound (vaginal probe) – the bladder, on the contrary, should be empty. This method gives a clearer image in the early stages and when evaluating the cervix. It is safe for pregnant women.

Explanation of the main indicators

You will have an ultrasound protocol with a number of abbreviations. The main ones are:
CTE — coccygeal-parietal size (in mm, first trimester).
BPD is the biparietal size of the head.
OG is the circumference of the head.
OJ is the circumference of the abdomen.
DB is the length of the hip.
PVP is the estimated weight of the fetus.
IAJ is the index of amniotic fluid.
TVP is the thickness of the nuchal space (first trimester).
Do not try to interpret the protocol yourself - the doctor evaluates the indicators in the aggregate and in dynamics. Small deviations from the "average norm" without other signs of pathology are almost always a variant of the norm.

When to see a doctor

Consult a doctor unscheduled if:
Bloody discharge or severe abdominal pain appeared.
You don't feel the fetal movements after 20 to 22 weeks for a few hours.
The doctor at the scheduled ultrasound revealed abnormalities and referred you for additional diagnostics - go through it without delay.

Frequently Asked Questions

Is it possible to do an ultrasound more often than three times?

Yes, if medically necessary. Frequent ultrasound scans do not harm the fetus - this has been confirmed by many years of research. However, without indications, additional studies are not needed: all significant data are obtained as part of three scheduled screenings.

When can I find out the sex of the child on ultrasound?

The sex of the fetus is determined on ultrasound from about 16-17 weeks with good accuracy. At the second screening (18-21 weeks), it is visible quite reliably. However, if the fetus "turned away" or tucked its legs, it may not be possible to determine the exact sex.

Conclusion

Three routine ultrasound scans during pregnancy — 11–14, 18–21 and 32–34 weeks — make it possible to track the development of the fetus, identify most structural and chromosomal abnormalities, assess the condition of the placenta and prepare for childbirth. Ultrasound is safe and informative. Do not skip routine screenings and consult a doctor with any alarming symptoms in between.

Bibliography

  1. ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2013;41:102-113. https://pubmed.ncbi.nlm.nih.gov/23280739/
  2. ISUOG Practice Guidelines: performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2011;37:116-126. https://pubmed.ncbi.nlm.nih.gov/21268051/
  3. ACOG Practice Bulletin No. 175. Ultrasound in Pregnancy. Obstet Gynecol. 2016. https://www.acog.org/clinical/clinical-guidance/practice-bulletin
  4. NICE. Antenatal care. NICE Guideline NG201, 2021. https://www.nice.org.uk/guidance/ng201
  5. Ministry of Health of the Russian Federation. Clinical recommendations: Normal pregnancy. Ministry of Health of the Russian Federation, 2023. https://cr.minzdrav.gov.ru/
This material is for informational purposes only and does not constitute medical advice and is not a substitute for medical advice. If a child's sleep is accompanied by alarming symptoms, a pronounced deterioration in health, problems with breathing, feeding or weight gain, it is necessary to consult a qualified medical specialist as soon as possible, and not rely only on information from the Internet.