Understanding Your Pregnancy Scan Report
A pregnancy scan report can feel like a different language the first time you read it. This guide explains the common terms, measurements and charts you’ll see on a maternity ultrasound report, what they mean for your pregnancy, and when to seek urgent medical attention. It’s written for people who have already had or are booking a private scan and want practical, clinically grounded explanation of the results — real-world, service-focused information from IUS London.
Contents
- What a typical pregnancy scan report contains
- Key measurements and what they mean (CRL, BPD, HC, AC, FL)
- How dating and growth are assessed (percentiles, plotted charts)
- Liquor assessment (AFI) and placenta notes
- Common findings and typical next steps
- Red flags — when to go to A&E
- From Our Practice [SUGGESTED — verify with clinician]
- Summary table for quick reference
- Further reading and references
- Book a private ultrasound scan in London (CTA)
What a typical pregnancy scan report contains
Most ultrasound reports use a consistent structure so clinicians can quickly find the important information. A typical report includes:
- Patient details and scan date.
- Scan indication (dating, routine anomaly check, growth scan, bleeding, reduced movements).
- Gestation by dates (last menstrual period — LMP) and by scan (ultrasound dating).
- Fetal viability and number (single, twin, etc.).
- Key biometric measurements and calculated gestational age.
- Estimated fetal weight (EFW) and percentile, often plotted on growth charts.
- Placenta position and grade.
- Amniotic fluid assessment (often noted as AFI or deepest pool).
- Any anatomical observations or concerns.
- Management advice and recommended follow-up.
The report will usually finish with the sonographer’s or reporting clinician’s name and a recommendation (for example, “routine obstetric follow-up” or “refer to consultant-led maternity services”). If you’re reading a private report from IUS London, you may also receive an explanation from the sonographer at the time of the scan — but a written report is the formal record.
Key biometric measurements and what they mean
Ultrasound measures fetal size at different gestations. Each measurement has a specific use.
- Used in the first trimester (generally up to 13+6 weeks).
- Measures the length from the top of the fetal head (crown) to the bottom of the torso (rump).
- CRL is the most accurate measurement for dating a pregnancy in early gestation; small differences in CRL can shift your estimated due date by days [1].
- CRL — Crown-Rump Length
- Measures the transverse diameter of the fetal head (across the two parietal bones).
- Commonly used in the second trimester and beyond as part of head size assessment.
- BPD — Biparietal Diameter
- Circumference of the fetal head. Useful when BPD alone is affected by head shape.
- HC — Head Circumference
- Measures the circumference of the fetal abdomen. It is sensitive to fetal nutritional status and growth.
- AC — Abdominal Circumference
- Length of the fetal femur (thigh bone). Helps assess skeletal growth and contributes to estimated fetal weight.
- FL — Femur Length
These measurements are used together to calculate an estimated fetal weight (EFW) and to plot where your baby sits on a growth chart — usually reported as a percentile.
How dating and growth are assessed (percentiles and charting)
Dating
- Early pregnancy dating relies on CRL. Because early growth is relatively uniform, CRL gives the most precise estimation of gestational age in the first trimester [1].
- After the first trimester, a combination of biometric measures (BPD/HC/AC/FL) is used to estimate gestation and weight. Variability increases with gestation; therefore ultrasound dating becomes less precise later in pregnancy.
Growth and percentiles
- Below the 10th percentile → Small for gestational age (SGA). May prompt follow-up scans and Doppler assessments.
- Above the 90th percentile → Large for gestational age (LGA). May prompt assessment for maternal diabetes and delivery planning.
- Fetal growth is evaluated by comparing your baby’s measurements or EFW against population reference charts.
- The percentile indicates where your baby falls relative to an expected distribution. For example, a measurement at the 50th percentile is exactly average; 10th percentile is smaller than 90% of peers.
- Common clinical thresholds:
- Growth is clinically more significant if the baby crosses percentiles between scans (e.g., from 50th to 10th) rather than a single isolated small or large measurement.
Different charts — different references
- Some units use population charts; others use customised charts (which account for maternal height, weight, ethnicity, parity) or international standards such as INTERGROWTH-21st. The type of chart may be noted on the report and can alter interpretation [2].
Liquor assessment — AFI and what it tells you
AFI stands for Amniotic Fluid Index. It’s a quantitative method commonly used in third-trimester scans to estimate total amniotic fluid by measuring the deepest vertical pool in each of four uterine quadrants and summing them.
- Normal: commonly 8–18 cm (method-dependent).
- Oligohydramnios (reduced fluid): AFI ≤5 cm (or deepest pocket <2 cm) — depends on local protocol.
- Polyhydramnios (excess fluid): AFI >24–25 cm.
- AFI ranges (general guide):
- Low or high AFI may be associated with fetal compromise, placental insufficiency, or maternal conditions (e.g., ruptured membranes, diabetes). Management generally involves repeat scans, Doppler assessment, and obstetric review.
Some reports use "deepest vertical pool" (maximum vertical pocket) instead of AFI. Your report should specify which method was used.
Common findings and typical next steps
- Continue routine antenatal care.
- If a private anomaly scan is normal but your community maternity record differs, discuss with your midwife/obstetrician.
- Normal report with measurements on expected percentiles
- Often requires follow-up growth scans (for example at 2–4 week intervals).
- Additional tests may include Doppler studies (umbilical artery, middle cerebral artery), maternal blood glucose testing, or consultant review.
- Slightly small or large but isolated findings
- Referral to a fetal medicine specialist for detailed imaging and counselling.
- May include genetic testing discussion depending on findings.
- Clear structural concerns on anatomy scan
- Consider investigation for placental insufficiency; may lead to increased surveillance and timed delivery planning.
- Reduced growth velocity (crossing centiles downward)
Always read the “recommendation” line near the end of the report — it explains the expected next steps (routine care, repeat scan, or urgent referral).
⚠ Red flags — when to go to A&E
If you experience any of the following, seek immediate assessment at A&E or your local maternity unit. These are urgent symptoms and not details to “wait and re-book” for routine follow-up:
- Heavy vaginal bleeding (soaking a pad in less than an hour) or large clots.
- Severe, unrelenting abdominal pain or cramping accompanied by light-headedness or fainting.
- Sudden, severe reduction or absence of fetal movements after 24 weeks’ gestation (or a significant change from your baby’s usual pattern) — act immediately if you are concerned.
- A sudden gush or continuous leakage of fluid (possible ruptured membranes).
- High fever (≥38°C / 100.4°F), especially with abdominal pain.
- Severe headache, visual disturbance, sudden swelling of face/hands, or right upper quadrant pain (possible pre-eclampsia signs).
If you are uncertain whether symptoms are urgent, contact your maternity unit or use NHS 111 (or your local emergency number) for advice. The sonographer’s report is not a substitute for urgent clinical assessment if symptoms arise.
From Our Practice — IUS London operational approach [SUGGESTED — verify with clinician]
At IUS London we standardise reporting to support rapid clinical decision-making for both patients and referring clinicians. Our suggested practice pattern (anonymised and operational) is:
- For all private growth scans performed after 24 weeks, we report BPD, HC, AC and FL, calculate EFW with Hadlock formula, and plot the EFW on an INTERGROWTH-21st and on a customised chart when maternal data are available. This dual-chart approach helps detect both population-level and customised deviations in growth and reduces false positives for SGA in women with small body habitus. [SUGGESTED — verify with clinician]
- If an AFI is outside normal ranges we include both AFI and deepest pool measurements and recommend immediate liaison with the patient’s maternity unit to ensure consistent follow-up thresholds.
- For early pregnancy scans where CRL is the primary dating measure, we document exact CRL to the nearest mm and provide the scan-derived estimated gestational age and expected delivery date, with a note about the expected dating accuracy (+/- days). If the CRL-dated EDD differs from LMP-derived EDD by >7 days in the first trimester, we recommend updating the maternity record.
These operational decisions are designed to make private scan results easy to act on by NHS or private obstetric teams. Please note that individual clinician practice and NHS trust protocols vary — verify with the responsible clinician for your care team.
Quick-reference summary table
| Measurement / term | What it measures | Typical use | Clinical significance if abnormal |
|---|---|---|---|
| CRL (crown–rump length) | Length from head crown to rump | First-trimester dating (≤13+6 wks) | Primary dating tool; large discrepancy with LMP may change EDD |
| BPD (biparietal diameter) | Head transverse diameter | Second trimester and later head size | Abnormal with head shape issues, growth restriction/overgrowth |
| HC (head circumference) | Head size and neurodevelopmental growth tracking | Helps detect microcephaly/macrencephaly | |
| AC (abdominal circumference) | Abdominal circumference | Nutritional status; component of EFW | Low AC → possible growth restriction; high AC → possible macrosomia |
| FL (femur length) | Fetal femur length | Skeletal growth; component of EFW | Short FL may indicate skeletal dysplasia or constitutional smallness |
| Percentile (EFW plotted) | Where EFW sits relative to reference population | Growth assessment | <10th → SGA (investigate); >90th → LGA (assess for diabetes) |
| AFI (amniotic fluid index) | Total amniotic fluid by quadrant sum | Third-trimester fluid assessment | Low AFI (oligo) → possible placental insufficiency; high AFI (poly) → maternal diabetes, fetal anomalies |
Practical tips for reading your report
- Check the gestation by scan line against your LMP-derived gestation. Early scans may reset your due date.
- Look for the percentile or phrasing such as “EFW at 30th centile” — that tells you where the baby sits on the growth curve.
- Note any recommendation: “Repeat growth scan in 2 weeks”, “Refer to consultant”, or “Routine follow-up” — these are the action points.
- If you see unfamiliar abbreviations (e.g., EFW, AFI, CRL), consult the glossary at the end of your report or ask your sonographer or midwife to explain.
- Keep a copy of your private scan report with your maternity notes so NHS midwives/obstetricians can review measurements and follow-up plans.
Frequently asked questions
Q: My report says “EFW 34 weeks (20th centile)”. Is that fine?
A: That typically indicates your baby’s estimated weight corresponds to what is expected at 34 weeks and sits at the 20th percentile — within normal limits. What matters more is growth trend: repeat scans may be advised to confirm stable growth.
Q: My AC is low but other measurements are normal. Should I worry?
A: Isolated small AC can reflect relative sparing of head growth in fetal adaptation to mild placental insufficiency or measurement variability. Your clinical team may recommend repeat growth scan and Doppler assessment.
Q: Can CRL dating change my due date?
A: Yes — CRL is the most accurate early gestational measurement. If ultrasound-dating differs significantly from LMP-derived dates in the first trimester, clinicians often use the scan estimate as the working due date [1].
Further reading and references
- National Institute for Health and Care Excellence (NICE). Antenatal care for uncomplicated pregnancies. NICE guideline NG201. https://www.nice.org.uk/guidance/ng201 [1]
- ISUOG (International Society of Ultrasound in Obstetrics and Gynecology). ISUOG Practice Guidelines: performance of the routine mid‑trimester fetal ultrasound scan. https://www.isuog.org/resource/isuog-practice-guidelines-performance-of-the-routine-mid-trimester-fetal-ultrasound-scan.html [2]
- NHS: Ultrasound scans in pregnancy — what to expect. https://www.nhs.uk/pregnancy/keeping-well/ultrasounds/ [3]
- INTERGROWTH-21st Project: International fetal growth standards (overview). The INTERGROWTH-21st Project. https://intergrowth21.tghn.org/ (see primary publications via The Lancet series) [4]
- Royal College of Obstetricians and Gynaecologists (RCOG). Green-top guideline summaries and patient information (see specific guidelines relevant to fetal growth and antenatal surveillance). https://www.rcog.org.uk/ [5]
(If you would like direct links to specific peer-reviewed articles on fetal biometry or Doppler studies used in fetal growth surveillance, we can provide a tailored reading list or PDF copies where available.)
Book a private ultrasound scan in London
If you have an ultrasound report you’d like explained in person or you’d like a private dating, anomaly or growth scan, IUS London offers specialist obstetric ultrasound services with experienced sonographers and consultant reporting. Book your private pregnancy scan today:
- Early Pregnancy and Dating Scan: /services/early-pregnancy-scan
- 12-week/Nuchal Translucency Scan: /services/12-week-nuchal-scan
- Anomaly (20-week) Scan: /services/anomaly-scan
- Growth and Doppler Scan: /services/growth-scan
- Book online or call us: /book
If you’re concerned by anything on your report, or if your report recommends urgent follow-up, contact your maternity unit immediately. For reassurance, or to arrange a private scan and a clear, clinician-led report you can take to your maternity team, book with IUS London today.
References
[1] NICE. Antenatal care for uncomplicated pregnancies. NG201. https://www.nice.org.uk/guidance/ng201
[2] ISUOG. Practice Guidelines: performance of the routine mid‑trimester fetal ultrasound scan. https://www.isuog.org/resource/isuog-practice-guidelines-performance-of-the-routine-mid-trimester-fetal-ultrasound-scan.html
[3] NHS. Ultrasound scan - pregnancy. https://www.nhs.uk/pregnancy/keeping-well/ultrasounds/
[4] INTERGROWTH-21st Project. Fetal growth standards. https://intergrowth21.tghn.org/
[5] Royal College of Obstetricians and Gynaecologists. https://www.rcog.org.uk/
Call to action: Ready to clarify your scan report or book a private ultrasound in central London? Book your appointment now at IUS London: /book
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