Use this tool to interpret miscarriage risk after a heartbeat is seen. Enter gestational age and fetal heart rate to see a risk band, a short trend to next week, and when to re-scan.
Overview of the estimate
- Baseline after heartbeat: risk generally falls as gestational age advances from about 6 weeks to about 10 to 12 weeks. We set a week by week educational baseline that decreases with time, consistent with guideline summaries of early pregnancy loss risk after heartbeat confirmation (ACOG Practice Bulletin: Early Pregnancy Loss; NICE NG126).
- FHR category by GA: fetal heart rate is classified as slow, borderline, normal, or very fast using GA adjusted cut points derived from early ultrasound studies that relate embryonic heart rate to viability (Radiology 1995, Doubilet & Benson; Radiology 2005, Benson et al.). Slow or borderline adds to the baseline; normal or very fast reduces or leaves it unchanged.
- Other modifiers: current bleeding, maternal age 35 or older with an extra step at 40, and assisted conception (IVF or IUI) apply small upward adjustments. These do not outweigh improvement if FHR normalises at a follow up scan (see NICE NG126 and patient safety advice from the NHS).
- Projection to next week: we show how risk usually drops one week later, assuming no new concerns. If FHR is expected to reach the next week’s normal threshold, the tool displays a typical reduction.
- Re-scan timing: suggested window depends on the band and FHR category, typically 7 to 14 days, aligning with early pregnancy ultrasound follow up practice (ACOG; NICE).
GA specific FHR thresholds used for the slider legend
- About 6 weeks: Slow <110 bpm; Borderline 100 to 109; Normal 110 to 159; Very fast ≥160.
- About 7 weeks: Slow <120 bpm; Borderline 110 to 119; Normal 120 to 169; Very fast ≥170.
- About 8 weeks: Slow <125 bpm; Borderline 115 to 124; Normal 125 to 169; Very fast ≥170.
- About 9 to 10 weeks: Slow <130 bpm; Borderline 120 to 129; Normal 130 to 179; Very fast ≥180.
Cut points are simplified from early pregnancy heart rate studies for readability and to provide safe brackets for counselling (Radiology 1995; Radiology 2005).
How the band is formed
- Start with baseline: a GA based baseline risk after heartbeat.
- Apply FHR effect: slow adds the largest penalty, borderline a smaller penalty, normal keeps baseline, very fast may slightly reduce the estimate.
- Add small modifiers: bleeding today, age thresholds, and IVF or IUI add small increments.
- Band label: the final percentage maps to Low, Moderate, or High and is shown with the percentage for clarity.
Pre heartbeat nuance
If FHR is very low for the stated GA or not clearly seen, we include a neutral note encouraging a repeat scan based on GA and cycle dates, reflecting that embryo visibility and FHR emergence depend on ovulation and implantation timing, not just calendar weeks (NICE NG126).
Follow up timing and safety
- Re-scan: typically 7 days when FHR is slow or the band is High, 10 days for Moderate, and about 14 days for Low.
- Red flags: heavy bleeding that soaks a pad in an hour, severe pain, one sided pain, dizziness or fainting, or fever should prompt urgent assessment regardless of any calculator estimate (NHS miscarriage guidance).
Limitations
- Different machines, sonographers, and embryo positions can shift measured heart rate by a few beats per minute.
- Gestational age is often estimated. Ovulation and implantation vary, especially with irregular menstruation or assisted reproduction.
- This is a simplified educational model. It does not replace clinical judgment, serial ultrasound, or combined biomarkers such as hCG and progesterone.
- ACOG Practice Bulletin: Early Pregnancy Loss – definitions, prognosis after heartbeat, and follow up principles.
- NICE NG126: Ectopic pregnancy and miscarriage – triage, ultrasound timing, and safety advice.
- Doubilet & Benson, Radiology 1995 – embryonic heart rate by gestational age and expected ranges in very early pregnancy.
- Benson et al., Radiology 2005 – association of slow embryonic heart rate with outcome and guidance for follow up.
- NHS: Miscarriage – patient facing overview of symptoms, when to seek help, and next steps.
How to use this calculator
- Enter the current gestational age in weeks and days.
- Enter the fetal heart rate in beats per minute.
- Optional: add bleeding today, maternal age, and whether conception was IVF or IUI.
- Read the current risk band and a note on how risk may change by the next scan if FHR is in the expected range for that week.
- See suggested re-scan timing and a red-flag reminder.
Educational only, not medical advice. Seek urgent care for heavy bleeding, severe or one-sided pain, dizziness, or fainting.
Frequently asked questions
Why do gestational age and FHR both matter?
Risk depends on how far along the pregnancy is and whether the heart rate fits the expected range for that week. Both pieces help frame short-term risk.
What is a “slow” fetal heart rate?
“Slow” is defined relative to gestational age. The result highlights caution when the entered FHR is below typical ranges for that week, with careful wording.
Does bleeding change the result?
Bleeding can influence monitoring. Adding bleeding today helps tailor re-scan timing and the red-flag reminder in your result.
If the heart rate improves, does risk fall?
When FHR normalises for the next scan, risk often drops. The result includes a simple “if FHR ≥ X next scan, risk ≈ Y%” style note.
What if there is no heartbeat yet?
Use the general Miscarriage Risk Calculator or speak with your care team about timing for a repeat scan. Early scans may miss a heartbeat before it is detectable.
When should I seek urgent care?
Seek urgent care for heavy bleeding, severe or one-sided pain, passing large clots, dizziness, or fainting.