Use this tool to interpret vanishing twin risk by week and twin type. It shows risk now, a short projection to next week, and when to seek care.
Scope and definitions
- Chorionicity: Di/Di means two placentas and two sacs, Mono/Di means one placenta and two sacs, Mono/Mono means one placenta and one sac. Chorionicity is central to risk assessment and surveillance (NICE NG137: Twin and triplet pregnancy; RCOG Green-top 51; Fetal Medicine Foundation).
- Heartbeat status: estimates are most informative after at least one heartbeat is seen. When neither heartbeat is confirmed, uncertainty is higher and repeat scanning is recommended per early pregnancy guidance (NICE NG126).
How the baselines are set
- By week: early loss is more common at 6–8 weeks and tapers by 10–12 weeks as ongoing pregnancies stabilise (StatPearls: Vanishing Twin Syndrome).
- By chorionicity: monochorionic twins have higher early risk and additional complications compared with dichorionic twins, so their baselines are set higher (RCOG Green-top 51; ACOG: Multifetal Gestations).
- Loss of one vs both: loss of one twin in early first trimester is more frequent than simultaneous loss of both, especially in Di/Di pregnancies (StatPearls; NICE NG137).
Heartbeat and other modifiers
- Both heartbeats seen: we apply a reduction to loss estimates, reflecting improved prognosis after cardiac activity is confirmed (general principle supported in early pregnancy viability guidance: NICE NG126).
- Only one heartbeat seen: we add a modest penalty because discordant findings carry more uncertainty for the co-twin without a heartbeat (observational patterns described in reviews such as StatPearls).
- Symptoms and background: current bleeding, higher maternal age, and assisted conception (IVF/IUI) are modeled as small upward adjustments consistent with general risk discussions in guidelines (ACOG; NICE NG126).
Projection to next week
When no new concerns arise, risk usually falls with advancing gestation. The calculator applies a proportional reduction to show an educational one-week trend (reassurance principle reflected across guideline narratives: NICE NG137).
Follow-up timing
- Typical interval: about 10 days for re-scan balances reassurance with practical scheduling in early twin pregnancies.
- Sooner review: about 7 days if a heartbeat is not seen for one or both twins or if the overall band is high, aligning with early pregnancy repeat-scan practice (NICE NG126).
Limitations
- Published data vary in inclusion criteria, scanning schedules, and definitions; this tool uses simplified bands rather than a formal survival model.
- Monochorionic-specific complications (for example, TTTS/TAPS) and later-pregnancy risks are not modeled here; clinical surveillance follows dedicated protocols (RCOG Green-top 51).
- Cord insertion type and placental location are not inputs; they are mentioned for awareness only.
- NICE NG137: Twin and triplet pregnancy – chorionicity-based care and surveillance.
- RCOG Green-top Guideline 51 – risks and management for monochorionic twins.
- ACOG Practice Bulletin: Multifetal Gestations – background on prognosis and risk factors.
- StatPearls: Vanishing Twin Syndrome – overview of incidence, timing, and outcomes.
- Fetal Medicine Foundation: Chorionicity and amnionicity – definitions and clinical importance.
- NICE NG126: Ectopic pregnancy and miscarriage – principles for repeat scanning and early pregnancy safety.
How to use this calculator
- Select the twin type: di/di (two placentas, two sacs), mono/di (one placenta, two sacs), or mono/mono (one placenta, one sac).
- Enter the current gestational age in weeks and days.
- Mark whether a heartbeat was seen for Twin A and for Twin B at this scan.
- Optional: add bleeding today, maternal age, and whether conception was IVF, IUI, or spontaneous.
- Read the estimated chance of losing one or both twins at this stage, the trend to next week, and when to contact your provider.
Educational only, not medical advice. Seek urgent care for heavy bleeding, severe or one-sided pain, dizziness, or fainting.
Frequently asked questions
What is a vanishing twin?
It describes the loss of one twin in early pregnancy, most often in the first trimester. The other twin may continue normally.
How does chorionicity affect risk?
Risk patterns differ for di/di, mono/di, and mono/mono twins because they share placentas and sacs differently. The calculator lets you select the type so results fit your scan.
What if only one heartbeat was seen?
If one twin has a heartbeat and the other does not, near-term risk and the week-ahead trend may differ. The result reflects the heartbeat status you enter for each twin.
Does bleeding change the result?
Bleeding can be common in twin pregnancies and may influence short-term monitoring. Add bleeding today so your result includes clearer guidance on re-scan timing and when to seek care.
Does IVF or IUI conception change risk?
Conception method can be part of clinical context. The tool captures this as an optional input and keeps the output educational, not diagnostic.
Can the surviving twin be affected?
Most early vanishing twin events do not harm the surviving twin. Care teams advise individual follow-up based on timing, chorionicity, and symptoms.
What about marginal or velamentous cord insertion?
These findings can matter clinically but are not modelled in this tool. They are best discussed with your clinician for tailored monitoring.
When should I seek urgent care?
Seek urgent care for heavy bleeding, severe or one-sided pain, passing large clots, dizziness, or fainting. If unsure, contact your care team.