How accurate are late third-trimester sc

Start here: tonight, spend about 20 minutes making one packet and one phone call — not re-reading the number on the scan.

1. Put the scan report, the pregnancy record, your ID, charger in one envelope by the door (include the maternity unit's daytime and night number, written out).
2. Write down one question: "What does the trend across my scans say, and what would make you offer induction — and what would make you offer it sooner?" That is the question that actually changes the decision.
3. Ring the maternity unit's antenatal/clinic line when it opens, or leave a message asking them to call you back. A tired caregiver's call gets answered better than a tired caregiver's silent email.

Do not let tonight's single weight estimate trigger a decision to induce. That is the part the ultrasound cannot do on its own.

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How accurate is the estimate, really?

• Ultrasound estimates fetal weight from measurements of the head, belly and leg bones. It is an estimate of a range, not a measurement.
• Published studies put the error at roughly 10–15% either way in the late third trimester, and wider than 20% in some groups. Treat these as estimates: I could only see the studies in search summaries this turn, not full papers, so ask your unit what error margin they treat as acceptable.
• Accuracy improves closer to term and is better when growth is normal. It gets less reliable when the baby is large, fluid is low, the baby is lying unusually, the placenta is in an awkward place, or earlier scans showed a different growth pattern.
• A practical way to feel it: at around 3 kg, 10–15% is roughly 300–450 grams in either direction — a wide band when someone asks "is this baby too big to deliver?"

Why one estimate should not decide induction

• Induction on the NHS may be advised for an overdue pregnancy, broken waters, or a health concern — and the timing of birth stays the pregnant person's informed choice. It is a decision about a whole picture, not about one number.
• The picture that matters: the trend across scans (not one scan), the relative size of the head and belly measurements, fluid levels, blood-flow findings if they were checked, blood pressure, past pregnancy history, and how the baby is moving now.
• A single estimate can be out by 10–20% in either direction, so it cannot by itself justify or refuse induction.
• One more reason to ask rather than assume: NHS England and the Royal College of Obstetricians and Gynaecologists published a patient safety alert in December 2025 about Intergrowth estimated fetal growth charts. The page I read was cut off before the full wording, so I cannot confirm the details — but it is worth asking which chart your unit is using and whether your estimate has been reviewed against it.

What to ask the maternity unit (any one of these opens the right conversation)

1. "Is my baby's growth tracking the same line as my earlier scans, or has the line changed?"
2. "Do my head and belly measurements look balanced, or is the baby growing unevenly?"
3. "What would you offer if the baby keeps growing on this line, and what would you offer if it drops?"
4. "What are my blood pressure and fluid readings today?"
5. "If we wait, when would you want to see me again, and what would change that plan?"

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Tonight / today (about 25 minutes total, built for a school-week evening)

Realistic success within 24 hours: a packed bag, a written question list, a named person, and the maternity team's own interpretation of the growth trend — not a promised delivery date and not a decision about induction.

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Decision tree

Call the maternity unit now (same day):

• The baby is moving less than usual, or she is not sure whether movements have changed.
• She has period-like cramps, regular tightenings, or pelvic pressure before 37 weeks.
• The scan report mentions something you do not understand and cannot get answered.

Call the maternity unit or local emergency services immediately, day or night — do not wait for the next appointment:

• Vaginal bleeding, or leaking fluid.
• Severe headache with blurred or flashing vision; pain high in the belly or under the right ribs; sudden puffiness of the face or hands.
• Severe or constant abdominal pain.
• Fast or difficult breathing, or chest pain.
• Fever with being too weak to get out of bed.
• A fit or seizure.
• Collapse, fainting, or confusion.
• One calf painful, red, or swollen.
• Anything visible or felt at the vaginal opening after the waters break.

Important: say the words "she is pregnant" and how many weeks — it changes who comes and how fast. Reduced movements are never something to sit on overnight; and "she was fine yesterday" is not reassurance, because some conditions give no warning first.

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Avoid list

• Do not decide induction (or refuse it) from one scan number, and do not compare her estimate with other people's babies.
• Do not start, stop, or change any medicine, supplement, or herb on the strength of a scan.
• Do not put anything in the vagina, and do not try to influence the estimate with food, herbs, or "induction" products bought online.
• Do not use a kick-count hour rule as a threshold for waiting — a change from her usual pattern is the thing to report.
• Do not let a general "big baby" worry turn into a plan you make without the maternity team.
• Do not spend tonight researching percentages on the internet; it will not move the decision.

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Official pages I read today (for you to open, or to quote at the clinic):
官方服务入口 (official service entry) — https://www.nhs.uk/pregnancy/your-pregnancy-care/ultrasound-scans/ · https://www.nhs.uk/pregnancy/labour-and-birth/inducing-labour/ · https://www.healthdirect.gov.au/procedures/diagnostic-tests-and-scans/ultrasound · https://www.gatesheadhealth.nhs.uk/resources/growth-scan-in-pregnancy/

These pages confirm that ultrasound is painless and uses no radiation, that induction may be advised for overdue pregnancy, broken waters or a health concern with the reason explained, and that a growth scan gives measurements that are plotted and reviewed by the team.

Source: who-iris https://iris.who.int/handle/10665/258716
Source: rcog https://www.rcog.org.uk/media/2mwncs1v/pi-your-babys-movements-in-pregnancy.pdf
Source: merck-pro https://www.merckmanuals.com/professional/gynecology-and-obstetrics/antenatal-complications/preterm-labor

*Limit: this is preparation, not diagnosis. The error ranges above are my best estimates from studies I could only see in summary form this turn; the alarm signs come from the pregnancy danger-sign list, and the decision about induction belongs to the maternity team with the pregnant person.*