Waiting for the last stretch and wondering what a third trimester ultrasound will actually tell you, and whether it will change your plan for birth. You want clarity, not jargon. You want to know when to schedule it, what it checks, what numbers matter, and how those results shape next steps if something unexpected appears. That is exactly what follows, from timing and preparation to growth charts, fluid thresholds, Doppler patterns, and how teams translate images into smart decisions.
What it is and how it differs from earlier scans
A third trimester ultrasound is a focused late pregnancy check. Earlier scans concentrate on dating and full anatomy. This one pivots to growth over time, amniotic fluid, placental position and appearance, fetal presentation, and tests of wellbeing that inform delivery timing. Think management, not a second anatomy survey. You might ask, will it feel different from my mid pregnancy scan. The flow is similar, the targets are different.
You will hear the broader family of terms too, like obstetric ultrasound, fetal ultrasound, and ultrasound scan. Some centers call it a growth scan or simply late pregnancy sonography.
Why and when to have it
- 28 to 32 weeks, set a late baseline for growth, amniotic fluid, and placenta, and catch early change.
- Around 32 weeks, a sweet spot that leaves time to adjust surveillance or delivery plans.
- 34 to 36 weeks, check presentation, fluid, and evolving findings.
- 36 to 38 weeks, tighten birth planning, confirm head down or breech, reassess fluid and placenta.
- 38 to 42 weeks, add scans or combined testing if there are post term concerns or new symptoms.
Not every pregnancy needs a routine third trimester ultrasound. It is most helpful when the result could steer care, for instance a size and date mismatch, reduced movements, bleeding, diabetes in pregnancy, maternal hypertension, twins, suspected growth problems, or placental concerns. High risk pregnancies may need serial scans every two to three weeks if fetal growth restriction (FGR) is suspected. When third trimester ultrasound shows steady growth, normal fluid, and a well positioned placenta, the plan often stays the same, which is a relief.
How the scan is done and safety
Preparation before the appointment
- No special diet or medicines are required.
- A full bladder is usually not needed this late.
- Avoid lotions or oils on the abdomen for one to two days prior, they can interfere with sound transmission.
- Wear clothing that lifts easily.
- Bring your referral, prior reports, and any recent labs that might be relevant.
Arrive a few minutes early. A partner or support person is often welcome, with limits based on space and privacy.
Who performs it and the technology used
A trained sonographer, obstetrician, or midwife acquires images. Decision making relies on two dimensional imaging, and Doppler when indicated. Three dimensional and four dimensional views may be offered if conditions are favorable, but the medical read leans on measurements and blood flow studies, not souvenir clips.
Duration and typical steps
Most third trimester ultrasounds take 20 to 30 minutes, sometimes longer with twins or a head tucked deep in the pelvis. A typical flow:
- Brief overview of what will be checked.
- Gel on the abdomen and a sweep across the uterus.
- Head, abdomen, and femur measurements to compute estimated fetal weight (EFW).
- Review of placenta, umbilical cord, and amniotic fluid volume.
- Confirmation of fetal presentation and lie.
- Doppler assessment of placental circulation when relevant.
- A verbal summary followed by a written report.
Curious to follow along. Ask the clinician to point out the head, the stomach bubble, the spine, the placenta. It can be reassuring and memorable.
Safety and precautions
Ultrasound uses sound waves, not ionizing radiation. Teams follow ALARA principles, as low as reasonably achievable, to use the minimum energy needed to answer the clinical question. There are no usual contraindications in late pregnancy, including with abdominal scars or twins. Prolonged entertainment only scans without medical oversight should not replace diagnostic imaging.
What the scan evaluates
Fetal growth and biometry
Third trimester ultrasound focuses on growth trend, not a single snapshot. The sonographer measures the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), then plots them on gestational age charts. Values between the tenth and ninetieth percentiles often fall in a physiological range, but context matters, parental size, prior growth curve, and time between scans all influence interpretation. Are you wondering what percentile really means. It is a ranking compared with babies of the same gestational age, not a grade for your baby.
Estimated fetal weight, methods and accuracy
EFW algorithms combine AC, HC, and FL, sometimes BPD. These formulas are validated, yet every estimate carries a typical error of about ten to fifteen percent. Accuracy can be reduced when the head is engaged, fluid is very low or very high, the abdominal wall is thick or scarred, or the baby keeps a tricky position. Third trimester ultrasound does not weigh a baby, it models weight from shape. That model still guides important decisions, especially when FGR or suspected large size are on the table.
- EFW helps when weighing the balance of continued maturation against risks of staying inside if the placental environment shows strain.
- Growth trends reassure when the curve stays steady, even if a single percentile number looks small.
If you like specifics, clinicians may also comment on fetal biometric measurements and report a fetal weight percentile.
Fetal wellbeing and the biophysical profile
Beyond measurements, clinicians watch movements, tone, breathing motions, and heart reactivity. A biophysical profile (BPP) combines ultrasound parameters, that is breathing motions, gross body movement, fetal tone, fluid, with heart rate reactivity. Some centers pair it with nonstress test (NST) or cardiotocography (CTG). High scores reassure. Borderline results prompt closer follow up. Low scores often trigger urgent evaluation or delivery planning depending on gestational age. You may hear terms like fetal breathing movements (FBM) or kick counts. Plainly put, the baby should flex, extend, and practice breathe, and the heart should respond to these bursts of activity. Parents can complement this with fetal movement counting (kick counts) at home.
Amniotic fluid assessment
Fluid can be measured by amniotic fluid index (AFI) or single deepest pocket (SDP). Thresholds used in many services:
- oligohydramnios: AFI less than 5 cm or deepest pocket less than 2 cm
- polyhydramnios: AFI above 24 to 25 cm or deepest pocket above 8 cm
Low fluid can hint at placental insufficiency or membrane rupture. High fluid can accompany gestational diabetes or fetal swallowing or intestinal issues. Both situations usually lead to closer checks and sometimes a change in delivery timing.
Placenta and umbilical cord
Reports document placental location, anterior, posterior, or fundal, and the distance from the cervix. Persistent placenta previa near term often requires planned cesarean. Suspicion of placenta accreta spectrum (PAS) prompts delivery planning in a center with surgical and neonatal support. Cord details may include cord insertion (velamentous) when visible. A nuchal loop around the neck is common and usually not harmful, but teams note it for awareness in labor. Some reports describe placental maturity, occasionally referred to as placental grading, placental maturity, or placental calcifications, which are descriptive and interpreted in context.
Fetal position and presentation
Third trimester ultrasound confirms fetal presentation and lie. Head down is cephalic presentation. Bottom first is breech presentation, which can be frank, complete, or footling. Sideways is transverse lie. Around 32 weeks many babies still rotate. If breech persists near 37 weeks, an external cephalic version, a hands on turn on the outside of the abdomen, may be offered when conditions are suitable.
Doppler and placental circulation
Doppler evaluates blood flow, a window into placental function. Common indices:
- umbilical artery Doppler for placental resistance
- middle cerebral artery Doppler for brain sparing adaptation
- uterine artery Doppler for maternal placental perfusion
- cerebroplacental ratio (CPR), the relationship of brain and placental flows
- Select centers add ductus venosus Doppler in complex growth restriction
Patterns that raise concern include rising umbilical resistance, absent or reversed end diastolic flow, high uterine artery resistance with notching, and a marked fall in cerebral indices. These, especially alongside FGR or abnormal fluid, lead to tighter monitoring and careful timing of delivery. Color maps, that is color Doppler, can be used to map vessels and cord insertion.
Cervical assessment when relevant
If preterm labor is suspected, a transvaginal scan may measure cervical length. After 28 to 30 weeks its predictive value falls, so it is not routine for all.
3D and 4D imaging
Three dimensional and four dimensional imaging can be lovely keepsakes and occasionally help visualize surface anomalies. Diagnostic decisions rely on two dimensional imaging and Doppler. Quality depends on fetal position, fluid in front of the face, maternal body habitus, and where the placenta sits.
Interpreting results and impact on the birth plan
Growth concerns, small and large babies
- FGR or small for gestational age, below the tenth percentile with slowed trajectory and or abnormal Doppler suggests placental insufficiency. Surveillance usually increases. If early delivery is likely, corticosteroids for lung maturity may be considered depending on gestational age. Third trimester ultrasound guides that call.
- Suspected large baby, EFW is imperfect. Decisions weigh shoulder dystocia risk against induction or cesarean risks, with diabetes status, pelvic factors, and labor history in the equation.
Amniotic fluid and placenta findings
Oligohydramnios near term often leads to closer testing or planning delivery. Significant polyhydramnios may prompt symptom relief, for instance posture or sleep adjustments, and planned timing if risks rise. Persistent previa or suspected accreta changes timing, mode, and setting of delivery and usually involves a center with surgical and neonatal backup.
Position, external cephalic version, and delivery options
If breech persists near 37 weeks and the conditions are favorable, external cephalic version may be offered. If it is not attempted or does not work, options include planned cesarean or, in selected units with specific expertise, vaginal breech birth. Third trimester ultrasound helps check the placenta location and cord position before a version attempt.
Twins and multiples
Management depends on chorionicity, whether twins share a placenta, growth discordance, and twin specific Doppler patterns. Timing of delivery and monitoring intervals are tailored, often with specialist input.
Dating and due date
Late scans rarely change the estimated due date unless there was no reliable early dating. Clinicians synthesize history, early scans, and current findings to guide care.
Special situations and limitations
Accuracy limits and what ultrasound cannot do
EFW has a usual error of about ten to fifteen percent. Image quality can be limited by position, fluid, scarring, or BMI. Some anomalies are subtle or develop after mid pregnancy. Ultrasound cannot detect many genetic or metabolic conditions. A normal third trimester ultrasound greatly lowers, but does not eliminate, the chance of structural problems.
Maternal and uterine factors
Higher BMI, abdominal scars, fibroids, or very low fluid can reduce clarity. These may lead to repeat scans or complementary tests. If you have questions, ask what might improve views, such as a short walk, a different maternal position, or a brief return visit.
Maternal medical conditions and complex pregnancies
Conditions such as diabetes, high blood pressure or preeclampsia, autoimmune disease, renal disease, or a prior stillbirth shape surveillance and timing. Complex findings often prompt coordinated planning with maternal fetal medicine and neonatal teams. Third trimester ultrasound is central to that choreography.
Practical logistics, coverage, and follow up
Coverage and costs
When there is a medical indication, many insurers cover late pregnancy imaging, though specifics vary by plan and region. For self pay scans, ask for an estimate and ensure the indication is documented.
After the scan, follow up and coordination
Your clinician will explain whether you need a repeat third trimester ultrasound, additional Doppler, a BPP, or specialist referral. Increased surveillance may include:
- More frequent ultrasounds for growth, Doppler, or fluid
- Fetal heart rate monitoring with NST or CTG
- Hospital admission when daily observation is needed, for example significant FGR or preeclampsia
Plans are built from multiple pieces, symptoms, blood pressure, kick counts, clinic assessments, and scan results. Together they support wise choices about induction or cesarean timing when needed. For reference, many services harmonize care with ACOG guidelines and NICE guidelines.
If an abnormality is found
Confirmatory tests and specialist referrals
Reasonable next steps include:
- Repeat ultrasound or Doppler to confirm a trend rather than a one off variation
- Fetal heart rate monitoring with NST or continuous CTG
- Fetal MRI for brain or spine detail in selected cases
- Fetal echocardiography if a heart concern is suspected
- Referral to a specialized prenatal center and planning with neonatal teams
The sequence balances medical urgency with time to understand and plan.
Communication and emotional support
Ask for a clear written summary you can read later. Keep a small list of questions on your phone, it helps in the moment. Seeking a second opinion is reasonable, particularly for complex or rare findings. Perinatal psychological support can help you process uncertainty and decisions without feeling rushed.
Practical arrangements
Depending on the situation, your team may suggest work leave, less travel, or hospital admission to support a safer end of pregnancy. Key information is documented in your maternity record so everyone stays aligned on the plan for birth and newborn care.
Questions to ask your provider
- How does my baby growth compare with prior scans, and what are the percentiles
- What is the estimated fetal weight (EFW) and the confidence range
- What is my baby presentation, and am I a candidate for external cephalic version if breech
- Where is the placenta, how far is it from the cervix, and is the umbilical cord insertion normal
- What is my amniotic fluid index (AFI) or single deepest pocket (SDP) and do I need follow up
- Do I need umbilical artery Doppler, middle cerebral artery Doppler, uterine artery Doppler, or a biophysical profile (BPP) based on today findings
- How do these results affect my timing and place of delivery
- If an intervention is suggested, what are the next steps and who coordinates care
Putting it all together, practical scenarios
- You have steady growth at the twentieth percentile, normal AFI, and normal Doppler. Third trimester ultrasound supports expectant management with routine visits and kick counts.
- You have AC lagging, low AFI, and elevated umbilical resistance. Third trimester ultrasound prompts closer surveillance, steroid consideration depending on timing, and planning for earlier delivery if the pattern persists.
- You have persistent breech at 37 weeks, a posterior placenta, normal fluid, and no contraindications. Third trimester ultrasound supports offering external cephalic version with appropriate monitoring.
- You have twin pregnancy with growth discordance and abnormal CPR in one twin. Third trimester ultrasound guides a twin specific plan with specialist input and more frequent checks.
Why parents search for a third trimester ultrasound, common questions answered
- Is it safe. Yes. Ultrasound uses sound waves, and teams follow ALARA to keep energy exposure low.
- Will I get 3D pictures. Sometimes, if the position and fluid cooperate. The medical read relies on two dimensional views.
- Can it change my due date. Rarely, unless no reliable early dating exists.
- Can it predict my birth weight exactly. No. It estimates, and clinicians pair that estimate with your history and the baby growth trend to guide decisions.
A quick note on language. You will see third trimester ultrasound called a late pregnancy growth scan in some settings. Others will refer to it simply as obstetric ultrasound. These are different labels for the same practical goal, a clear look at growth, fluid, placenta, presentation, and circulation as you approach birth.
Key takeaways
- A third trimester ultrasound focuses on growth, presentation, placenta, fluid, and wellbeing. It informs safe, flexible birth planning.
- A common timing is 31 to 33 weeks, and there may be additional checks at 34 to 36 and 36 to 38 weeks if needed.
- EFW has a typical error of about ten to fifteen percent. Trends and clinical context matter more than one number.
- Fluid thresholds matter, AFI less than 5 cm or deepest pocket less than 2 cm suggests oligohydramnios, AFI above 24 to 25 cm or deepest pocket above 8 cm suggests polyhydramnios.
- Doppler of the umbilical, uterine, and middle cerebral arteries can flag placental stress and guide timing.
- Medically indicated ultrasounds are safe and follow ALARA. Keepsake scans should not replace diagnostic imaging.
- Clear communication, coordinated planning, and compassionate support help parents make confident decisions.
- For tailored advice and handy health tools, download the application Heloa for personalized tips and free child health questionnaires.
Third trimester ultrasound appears many times in this page because it is the anchor of late pregnancy surveillance, and because the right scan at the right time can turn anxiety into informed action. If you still have questions, bring them to your next visit and ask how your third trimester ultrasound will be used to shape a plan that fits your family.
Questions Parents Ask
How many ultrasounds will I need in the third trimester?
It depends on your pregnancy. Many low‑risk pregnancies need no routine late scan or only a single check around 32–37 weeks to confirm presentation and growth. If there are concerns—diabetes, high blood pressure, prior growth issues, twins, or any abnormal findings—scans may be repeated every 1–3 weeks or as directed by your care team. The frequency is chosen to balance reassurance with medical need: more scans when a trend must be watched closely, fewer when things are stable. If you feel anxious about timing, ask your provider what pattern they expect and how each scan would change care.
Can a third‑trimester ultrasound reliably tell me the baby’s sex?
Often yes, but not always. The baby’s position, low fluid, or fetal head engagement can hide the genital area late in pregnancy. When the anatomy is visible, sex identification is usually accurate, but visibility—not the machine—limits certainty. Remember some clinics focus only on medical checks and may not volunteer or prioritize sex determination; feel free to ask before the scan if you want to know and whether the sonographer can try to look.
If the scan shows a concern, how quickly will I be informed and what happens next?
You will usually get an immediate verbal summary at the appointment and a written report soon after. Urgent abnormalities are typically communicated the same day and trigger next steps like a repeat scan, Doppler studies, a biophysical profile or fetal monitoring (NST/CTG) within hours to days depending on severity and gestational age. Some situations lead to referral to a maternal‑fetal medicine specialist, fetal MRI, or discussion of earlier delivery; others mean closer outpatient surveillance. If you want clarity, ask at the scan who will call you, expected timelines for any follow‑up, and who to contact if you notice reduced fetal movements or new symptoms. This helps you feel supported and prepared while clinicians sort the best plan.
Further reading :
