How this instrument works
The RCOG Green-top Guideline No. 37a (3rd edition, April 2015), Appendix III, is a single unified point table for estimating a pregnant or postpartum woman's venous thromboembolism (VTE) risk — the same table is reused at every assessment timepoint (booking, any antenatal hospital admission, 28 weeks, and again after delivery), rather than there being separate antenatal and postnatal tables as is sometimes assumed. Values are summed across three categories — pre-existing factors such as age, BMI, and personal or family history; obstetric factors specific to the current pregnancy or delivery; and transient factors that can appear or resolve over time — and the running sum is what this calculator reports as the Appendix III total.
The guideline turns that total into action through threshold rules, with one important carve-out. Four or more threshold-counted risk factors antenatally means considering prophylaxis from the first trimester; exactly three means considering it from 28 weeks; two or more postnatally (without reaching an antenatal threshold) means considering at least 10 days of postnatal prophylaxis. Critically, the guideline's own text excludes previous VTE and high-risk thrombophilia from that generic threshold count — 'any woman with four or more current risk factors... other than previous VTE or thrombophilia' — because those two factors instead route to a dedicated specialist-referral pathway (Appendix IV) regardless of the number they contribute. This calculator reports both a raw total (the literal Appendix III sum) and a separate threshold figure that subtracts previous-VTE and high-risk-thrombophilia values before applying the 4/3/2 rules, plus a specialist-referral flag and a flag for extending postnatal prophylaxis to at least 6 weeks — triggered by reaching either antenatal tier (the guideline's own executive summary states both the 4+ and the 3-point antenatal groups 'will usually require' 6 weeks postnatally, not just the default 10 days), by specialist-referral criteria, or by low-risk thrombophilia and a family history of VTE occurring together.
Two things in the source material deserve to be named directly rather than quietly resolved. First, RCOG's own guidance page carries an erratum stating a correction was made to link dehydration with hyperemesis rather than with immobility, and the currently hosted PDF of the guideline reflects that correction — its Appendix III table lists 'Hyperemesis, dehydration' combined as one 3-value item and 'Immobility' alone as a 1-value item. This calculator follows that corrected, currently-live pairing: its 'Hyperemesis or dehydration' field is worth 3 points, and a separate 'Immobility' field is worth 1 point on its own. Second, the obesity row's BMI comparator is not actually ambiguous in the source: Appendix III's footnote reads 'BMI ≥ 30 = 1; BMI ≥ 40 = 2' in plain, correctly encoded text, corroborated elsewhere in the same guideline by 'Obesity (BMI ≥ 30 kg/m2)' in Table 1 and by 'class 3 obesity (BMI greater than or equal to 40 kg/m2)' in the body text — so the BMI 30/40-or-above cutoffs used here are a direct transcription, not an interpretation of an illegible character.
This is a clinician-administered assessment tool, not a one-shot self-assessment — the guideline explicitly frames VTE risk in pregnancy as something to re-evaluate at multiple visits, since transient factors like hyperemesis, prolonged immobility, or a systemic infection can appear or resolve as pregnancy and the puerperium progress. As with any RCOG Green-top guideline, its recommendations are meant to be weighed against individual patient circumstances rather than applied as a mandatory, one-size-fits-all rule, and a numeric result alone does not capture everything relevant to a prophylaxis decision — bleeding-risk factors, for instance, can outweigh a high result and should be discussed with a haematologist before anticoagulation starts.
- Enter Age (age), Height (height) in metres, and Weight (weight) in kilograms — the calculator derives your BMI automatically for the obesity value.
- Answer Previous VTE (previousVte) and Known high-risk thrombophilia (highRiskThrombophilia) with care — both add to the raw total but instead drive the specialist-referral flag, not the thresholds.
- Answer the remaining pre-existing factors: medical comorbidity (medicalComorbidity), family history of VTE (familyHistoryVte), low-risk thrombophilia (lowRiskThrombophilia), smoker, and varicose veins (varicoseVeins).
- Answer the obstetric factors for this pregnancy or delivery: pre-eclampsia, ART/IVF, multiple pregnancy, caesarean in labour, elective caesarean, operative delivery, prolonged labour, PPH, preterm birth, and stillbirth.
- Answer the transient factors right now — surgical procedure, hyperemesis/dehydration, OHSS, infection, immobility — then repeat as these can change.
- Review the raw total, the threshold figure, the antenatal and postnatal tiers, and the specialist-referral and extended-6-week flags shown with the breakdown.
Worked example: previous VTE and thrombophilia trigger referral
A 40-year-old with a BMI of 41.0 (weight 105 kg, height 1.60 m) and a parity of 3 earns 1 for age over 35, 2 for BMI 40 or above, and 1 for parity 3 or above. She also has a previous VTE not related to major surgery (4), known high-risk thrombophilia (3), a qualifying medical comorbidity (3), a family history of VTE (1), current smoking (1), and gross varicose veins (1) — a pre-existing subtotal of 17.
There are no obstetric factors in this pregnancy (0), but two transient factors apply: a non-perineal surgical procedure during pregnancy (3) and a current systemic infection (1), adding 4 more. Summing 17 + 0 + 4 gives a raw Appendix III total of 21 — the literal 'TOTAL' row of the source table.
Because the guideline excludes previous VTE and high-risk thrombophilia from the generic threshold rule, the calculator subtracts those two contributions (4 and 3) from the raw total: 21 − 7 = 14. That threshold figure is well above 4, so the antenatal tier is 2 (consider prophylaxis from the first trimester) and the postnatal tier is 1 (at least 10 days postnatally). But it is the previous VTE and high-risk thrombophilia themselves, not the threshold figure, that trigger specialist referral — and because referral criteria are met, postnatal prophylaxis extends to at least 6 weeks rather than the default 10 days.
Questions
Is this the same point table used at every stage of pregnancy?
Yes — RCOG Green-top Guideline No. 37a, Appendix III, uses a single unified table that is reused at booking, at any antenatal hospital admission, at 28 weeks, and again intrapartum and postpartum, rather than separate antenatal and postnatal tables. What changes between assessments is which factors currently apply and whether the 4/3-point antenatal thresholds or the 2-point postnatal threshold is being checked, not the underlying values themselves.
Why don't previous VTE and high-risk thrombophilia count toward the thresholds?
The guideline's own text excludes them explicitly: 'any woman with four or more current risk factors... other than previous VTE or thrombophilia' should be considered for prophylaxis from the first trimester. Previous VTE and high-risk thrombophilia instead route to a dedicated specialist-referral pathway in RCOG Appendix IV, because these two factors carry a level of risk the generic threshold rule was not designed to capture — either one should prompt referral to a specialist thrombosis-in-pregnancy team regardless of the numeric result.
What's the difference between the raw total and the threshold figure?
The raw total is the literal sum of every row in the Appendix III table, including previous VTE and high-risk thrombophilia — it is what the source document's own 'TOTAL' row would show. The threshold figure subtracts previous-VTE and high-risk-thrombophilia contributions from that raw total, since those two are excluded from the generic threshold rule and handled through specialist referral instead. This calculator reports both so the arithmetic and the threshold decision stay traceable to the source table.
When does postnatal prophylaxis extend to 6 weeks instead of 10 days?
Three situations extend postnatal prophylaxis from the default 'at least 10 days' to 'at least 6 weeks': reaching either antenatal tier (the guideline's executive summary states both the 4+-point and 3-point antenatal groups 'will usually require' 6 weeks postnatally); specialist-referral criteria being met (previous VTE or high-risk thrombophilia); or low-risk thrombophilia plus a family history of VTE applying together. The plain 10-day default is reserved for women who reach 2 threshold-counted points only at a postnatal (re)assessment, without having met an antenatal threshold.
Why does 'hyperemesis or dehydration' combine into one field, with immobility separate?
RCOG's own guidance page carries an erratum stating a correction was made to link dehydration with hyperemesis, not immobility, and the currently hosted PDF of the guideline reflects that corrected pairing: 'Hyperemesis, dehydration' combined for 3 points, and 'Immobility' alone for 1 point. This calculator follows that corrected, currently-live pairing directly — its 'Hyperemesis or dehydration' field is worth 3 points, and a separate 'Immobility' field is worth 1 point on its own, matching the source table as it stands today rather than the older, since-corrected version.
How does the calculator decide the obesity BMI cutoffs?
Appendix III's obesity row carries a footnote that reads, in full, 'BMI ≥ 30 = 1; BMI ≥ 40 = 2' — a plain, correctly encoded comparator with no illegible or ambiguous character. That reading is corroborated elsewhere in the same guideline: Table 1 lists 'Obesity (BMI ≥ 30 kg/m2)' as a risk factor, and the body text separately describes 'class 3 obesity (BMI greater than or equal to 40 kg/m2)' in full words. This calculator uses that directly confirmed transcription: BMI 30 or above scores 1, and BMI 40 or above scores 2.
How often should this assessment actually be repeated?
The guideline treats VTE risk assessment as an ongoing process rather than a single figure captured once. RCOG recommends assessing risk at booking, on any antenatal admission to hospital, at 28 weeks, and again intrapartum or immediately postpartum, because transient factors — hyperemesis, immobility, a surgical procedure, a systemic infection — can develop or resolve at any of those points. Repeating this assessment at each moment, rather than relying on a single early-pregnancy figure, is how the tool is meant to be used.
Can this calculator make a prophylaxis decision on its own?
No. It reproduces the RCOG Appendix III arithmetic and threshold rules and nothing else — it does not see bleeding-risk factors, renal function, platelet count, or the many other clinical details a haematologist or obstetrician weighs alongside the result. RCOG is explicit that its guideline recommendations must be evaluated against individual patient needs and are not mandatory or exclusive of other approaches; a result that supports prophylaxis can still be overridden by a bleeding risk that needs specialist discussion before anticoagulation starts.
References
- RCOG Green-top Guideline No. 37a — Reducing the Risk of VTE during Pregnancy and the Puerperium
- RCOG Green-top Guideline No. 37b — Thromboembolic Disease in Pregnancy: Acute Management
- RCOG patient information — Venous thrombosis in pregnancy and after birth
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.