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Prenatal Guidance

Gestational age and due date by LMP, ART dates or ultrasound, with the antenatal recommendations for that gestational age compiled from WHO, national authorities and US societies — each body side by side, every statement cited

Tool

Also available as a window in the Workspace. Everything runs in your browser; nothing is uploaded.

Why use it

Dating determines the timing of every antenatal test, the interpretation of screening results and the timing of birth. National schedules differ in windows, populations and tests, and they change: several were revised in 2023–2026. Clinicians caring for high-risk pregnancies, and those who follow women across health systems, need the applicable windows for a given gestational age together with the differences between guidelines. Prenatal Guidance keeps each recommendation tied to the exact sentence of its source, so it can be checked and documented.

About

Prenatal Guidance calculates gestational age and the estimated due date (EDD) from the last menstrual period, from IVF transfer or IUI dates, and from ultrasound biometry (crown–rump length, head circumference with or without femur length). It applies the best-obstetric-estimate approach: IVF dates are never revised by ultrasound; otherwise the ultrasound estimate replaces the LMP when the difference exceeds the ACOG Committee Opinion 700 threshold for the gestational age at the scan, or always when the UK/ISUOG convention is selected. Crown–rump length is converted with the Robinson–BMUS equation by default, or with the INTERGROWTH-21st equation; head circumference with the INTERGROWTH-21st or BMUS (Altman & Chitty) equations. For the gestational age on the report date, the tool lists antenatal examinations, screening tests, vaccinations, supplements and counselling compiled from the World Health Organization, national authorities (Brazil, Portugal, Germany, France, Italy, Spain, United Kingdom), US bodies (ACOG, SMFM, USPSTF, CDC) and international societies (ISUOG, FIGO, ISSHP, ISPD). Each item appears once; inside it, the position of every body that addresses it — including bodies that recommend against it — is shown side by side with its window, population and the verbatim text of the source. Where bodies disagree, the tool says so and does not choose. Risk factors and diagnoses entered in the Context tab switch on risk-based recommendations and condition modules (hypertensive disorders, diabetes, fetal growth restriction, preterm birth, twins, alloimmunisation, cholestasis, thyroid disease, congenital infections and others), with screening, diagnosis, surveillance, treatment and timing of birth, and a prenatal-genetics module (cell-free DNA, combined and serum screening, invasive testing, chromosomal microarray and sequencing, soft markers, carrier screening).

When to use

  • Assigning gestational age and the due date at booking or after the dating scan
  • Deciding whether an ultrasound result should change the due date (ACOG CO 700 thresholds or ultrasound-always convention)
  • Dating IVF (day-3, day-5, day-6/7 embryo) and IUI pregnancies, and twin pregnancies (larger CRL / HC)
  • Planning the tests, vaccinations and scans due now and in the coming weeks
  • Comparing what different national and international bodies recommend for the same item
  • High-risk pregnancies: condition-specific surveillance, treatment and timing of birth; prenatal genetic screening and diagnosis

Method

1. ART: EDD = transfer date + (266 − embryo age in days) [day 5: +261, day 3: +263, ACOG]; retrieval date + 266 (derived). 2. LMP: EDD = LMP + 280 days [+ (cycle − 28) days if the optional adjustment is selected]. 3. Ultrasound GA at the scan: Robinson–BMUS GA(d) = 8.052 × √(CRL × 1.037) + 23.73 CRL 5–84 mm INTERGROWTH GA(d) = 40.9041 + 3.21585 × √CRL + 0.348956 × CRL CRL 15–95 mm INTERGROWTH ln GA(d) = 0.03243 (ln HC)² + 0.001644 FL ln HC + 3.813 BMUS ln GA(wk) = 0.010611 HC − 0.000030321 HC² + 0.43498×10⁻⁷ HC³ + 1.848 4. Choice: IVF date always; else earliest adequate scan vs LMP/IUI date — ACOG CO 700: redate if |difference| > 5 d (≤ 8+6), 7 d (9+0–15+6), 10 d (16+0–21+6), 14 d (22+0–27+6), 21 d (≥ 28+0); or ultrasound always (UK/ISUOG). 5. GA on the report date = report date − (EDD − 280 days), in completed weeks + days. 6. Timeline: an item is "due now" when the report GA lies in the window of at least one body that recommends it for this woman.

Pearls & pitfalls

  • The EDD is assigned once, from the most accurate method available, and is not changed by later scans except in rare circumstances; later biometry assesses growth (ACOG CO 700; ISUOG 2019).
  • Crown–rump length dates most accurately in the first trimester; above 84 mm, head circumference is used (ACOG CO 700; ISUOG 2019; BMUS 2011).
  • IVF pregnancies are dated from the embryo age and transfer date: transfer + 261 days for a day-5 embryo, + 263 days for a day-3 embryo (ACOG CO 700). Day-6/7, oocyte-retrieval and IUI formulas are derived from the same convention and are labelled as such.
  • The cycle-length adjustment of the LMP due date is offered as an option but is not recommended by any of the guidelines consulted.
  • In spontaneous twins, gestational age is assigned from the larger CRL, and from 14 weeks from the head circumference of the larger twin (ISUOG 2025).
  • A pregnancy without an ultrasound that confirms or revises the EDD before 22+0 weeks is suboptimally dated (ACOG CO 700).
  • Recommendations not yet verified against the primary text are withheld from display; the count is shown in the Timeline tab.
  • Legal gestational-age limits are not encoded; they vary by jurisdiction.
  • Nothing is sent to a server: the record lives in the browser tab and in the downloaded file.

Next steps

Use the Timeline tab for items due now, in the next four weeks, at first contact and at every visit; check items whose window has passed. Switch on condition modules in the Context or Conditions tab. Record results per item and download the .prenatal.json file to continue at the next visit. Apply the local protocol where it differs.

Evidence

Sources are guidelines, consensus statements and national programme documents, read on 7–8 October 2026, each with its version and status (current, reaffirmed or superseded). Dating: ACOG Committee Opinion 700 (2017, reaffirmed 2025), ACOG CO 688 and 579, ACOG/SMFM Obstetric Care Consensus 6, BMUS (Loughna et al. 2009; Chudleigh et al. 2011), INTERGROWTH-21st (Papageorghiou et al. 2014, 2016), ISUOG 11–14-week (2023), biometry (2019) and twin (2025) guidelines, FIGO 2021, WHO ICD-10 and WHO preterm-birth definitions. Antenatal care: WHO 2016 recommendations and later updates; NICE NG201 and related NICE guidance; NHS screening programmes; ACOG, SMFM, USPSTF and CDC documents; Brazil Ministério da Saúde (Caderno 32, Manual de Gestação de Alto Risco 2022, PCDT and technical notes); Portugal DGS (programme 2015, Norma 001/2023, diabetes consensus 2017); Germany G-BA Mutterschafts-Richtlinien (as amended 2026); France HAS; Italy ISS-SNLG (2011; 2023–2026); Spain Ministerio de Sanidad and SEGO. The compilation method (one item per clinical action, every body side by side, no ranking) is a decision of this project, not a published recommendation. The tool has not been validated as a clinical decision instrument.

References

  1. American College of Obstetricians and Gynecologists, AIUM, SMFM. Methods for Estimating the Due Date. Committee Opinion No. 700. Obstet Gynecol. 2017;129:e150-e154 (reaffirmed 2025). — link
  2. Loughna P, Chitty L, Evans T, Chudleigh T. Fetal size and dating: charts recommended for clinical obstetric practice. Ultrasound. 2009;17(3):161-167. — link
  3. Papageorghiou AT, et al. International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester of pregnancy. Ultrasound Obstet Gynecol. 2014;44:641-648. — link
  4. Papageorghiou AT, et al. Ultrasound-based gestational-age estimation in late pregnancy. Ultrasound Obstet Gynecol. 2016;48:719-726. — link
  5. ISUOG Practice Guidelines (updated): performance of 11-14-week ultrasound scan. Ultrasound Obstet Gynecol. 2023;61:127-143. — link
  6. Khalil A, et al. ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy. Ultrasound Obstet Gynecol. 2025. — link
  7. World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience. Geneva: WHO; 2016. — link
  8. National Institute for Health and Care Excellence. Antenatal care. NICE guideline NG201. 2021. — link
  9. Society for Maternal-Fetal Medicine. Consult Series #74: cell-free DNA screening for fetal aneuploidies. 2025; ACOG Practice Advisory, January 2026. — link
  10. Brasil. Ministério da Saúde. Manual de Gestação de Alto Risco. Brasília; 2022. — link
  11. Direção-Geral da Saúde. Programa Nacional para a Vigilância da Gravidez de Baixo Risco. Lisboa: DGS; 2015. — link