Pregnancy Dating & Antenatal Scheduler

Evidence-Based Gestational Dating & ANC Scheduling · v1.8
How to use this tool: Enter the dating parameters. The tool returns a chronological clinical timeline, the vaccination schedule, and the Panchanga data (tithi and nakshatra) for the estimated date of delivery.

1. Gestational Dating Parameters

2. Geographical Delivery Location

Required to calculate local sunrise for the approximate Panchanga (Tithi & Nakshatra) on the EDD.

Core Principles of Gestational Dating

Accurate gestational dating is the cornerstone of safe obstetric practice. It determines the timing of aneuploidy screening, informs decisions surrounding preterm labour management, and dictates induction pathways for post-term pregnancies. Gestational age is always counted from the first day of the LMP, NOT from the date of conception.

Ultrasound vs. LMP Supremacy: While this tool uses the modified Naegele's rule based on LMP, ACOG and FOGSI guidelines state that an early first-trimester ultrasound (measuring Crown-Rump Length) is the most accurate method to establish or confirm gestational age. If ultrasound dating differs significantly from LMP dating, the EDD should be revised.

4. Naegele's Rule: The Mathematics

Standard formula (28-day cycle): EDD = LMP + 280 days (i.e., LMP + 9 months + 7 days).
Modified formula (irregular cycle): EDD = LMP + 280 + (Cycle Length - 28) days. This corrects for the variability in the follicular phase while assuming a constant 14-day luteal phase.

5. Mnemonic: "DATES" for Accurate Gestational Dating

D - Document the LMP precisely (first day of last period, NOT the last day)
A - Assess cycle regularity (irregular cycles need ultrasound dating)
T - Trust the earliest ultrasound (first trimester CRL is gold standard)
E - Evaluate discrepancy (redate if ultrasound differs beyond threshold)
S - Single EDD only (once established, do NOT change unless clinically mandated)

6. When to Redate: Ultrasound vs. LMP Discrepancy Thresholds

Ultrasound Timing Measurement Discrepancy Requiring EDD Redating
< 9+0 Weeks Crown-Rump Length (CRL) > 5 days variance from LMP
9+0 to 13+6 Weeks Crown-Rump Length (CRL) > 7 days variance from LMP
14+0 to 15+6 Weeks BPD, HC, AC, FL > 7 days variance from LMP
16+0 to 21+6 Weeks Composite Biometry > 10 days variance from LMP
22+0 to 27+6 Weeks Composite Biometry > 14 days variance from LMP
≥ 28+0 Weeks Composite Biometry > 21 days variance from LMP

Indian Clinical Context & Imperatives

7. MoHFW ANC Prophylaxis Protocols

Td Vaccine (NOT TT): The Ministry of Health and Family Welfare has replaced TT (Tetanus Toxoid) with Td (Tetanus + low-dose adult Diphtheria) to counter waning diphtheria immunity in adults. Dose 1 should be given early in pregnancy, and Dose 2 at least 4 weeks after Dose 1.
Iron and Folic Acid - Timing Matters: Folic Acid alone (400 to 800 mcg/day) is mandatory from the periconceptional period through the first trimester for neural tube defect prevention. The combined IFA tablet (100 mg elemental iron + 500 mcg folic acid) is started only from the second trimester onwards, as first-trimester iron supplementation may worsen nausea and provides no proven benefit for NTD prevention.
Statutory Compliance (PC-PNDT Act, 1994): Under the Pre-Conception and Pre-Natal Diagnostic Techniques Act, completion of Form-F and signed consent declaring that sex determination will not be performed is legally mandated prior to every obstetric ultrasound in India, not just the morphological scan. Non-compliance is a criminal offence.

8. GDM Screening: Dual Screening is Now the Standard

FOGSI 2024 Recommendation: All pregnant women should be screened TWICE for GDM during ANC. The first test should be performed at the first antenatal contact (as early as possible), and the second at 24 to 28 weeks if the first was negative.

DIPSI Single-Step Test: 75g oral glucose load irrespective of fasting status. A 2-hour plasma glucose value ≥ 140 mg/dL is diagnostic for GDM. Note: Recent evidence (WINGS study) has raised concerns about the low sensitivity (27.7%) of the non-fasting DIPSI test. Where feasible, a fasting 75g OGTT using IADPSG criteria offers superior sensitivity.

9. GBS Screening: Indian Context

Not part of Indian National ANC Protocol: Universal GBS screening (36+0 to 37+6 weeks) is an ACOG/CDC recommendation. India does not have a national GBS screening programme. Indian studies report GBS colonisation prevalence of 7 to 13% in tertiary centres. This tool includes the milestone for completeness, but GBS screening should be offered based on institutional protocol and available resources, not as a universal mandate.

Illness Scripts: Size-Dates Discrepancy

Feature Symmetrical FGR (Early Onset) Asymmetrical FGR (Late Onset) Macrosomia (Large for Dates)
Pathology Intrinsic foetal insult reducing overall cell number (hypoplasia). Uteroplacental insufficiency causing glycogen depletion in liver (hypotrophy). Maternal hyperglycaemia causing foetal hyperinsulinaemia (hypertrophy).
Aetiology Chromosomal anomalies (Trisomy 18, 13), TORCH infections, severe malnutrition. Pre-eclampsia, chronic hypertension, severe anaemia, smoking. Gestational Diabetes Mellitus (GDM), maternal obesity, multiparity.
Onset First trimester (early, global insult). Third trimester (late, selective insult). Second to third trimester (metabolic-driven).
Biometry HC and AC proportionally reduced. Normal HC/AC ratio. Brain-sparing: HC normal, AC severely reduced. High HC/AC ratio. Accelerated AC growth (liver hypertrophy). EFW > 90th centile.
Prognosis Poor. Often associated with structural or genetic abnormalities. Better if detected early. Improves with uteroplacental blood flow optimisation. Good if glucose controlled. Risk: shoulder dystocia, birth injuries.

Indian ANC Schedule: Minimum Visit Framework

MoHFW: Minimum 4 ANC visits (first in first trimester). FOGSI 2024: 10 to 12 visits recommended (monthly until 28 weeks, fortnightly until 36 weeks, weekly until delivery). WHO 2016: Minimum 8 ANC contacts.

10. Quick Reference: ANC Visit Checklist Mnemonic "ABCDEFGH"

A - Anaemia screening (Hb at booking, 28W, 36W)
B - Blood pressure at every visit
C - Clinical examination (fundal height, foetal heart)
D - Diabetes screening (first contact + 24 to 28W)
E - Ensure Td vaccination (2 doses)
F - Folic acid / IFA supplementation
G - Growth monitoring (serial symphysio-fundal height)
H - High-risk identification (pre-eclampsia, FGR, malpresentation)

11. Clinical Pearl: Term Pregnancy Stratification (ACOG)

Classification Gestational Age Clinical Significance
Early Term 37+0 to 38+6 weeks Higher NICU admission risk vs. full term. Elective delivery NOT recommended.
Full Term 39+0 to 40+6 weeks Optimal window. Lowest neonatal morbidity and mortality.
Late Term 41+0 to 41+6 weeks Increased risk of meconium aspiration, macrosomia. Consider induction.
Post-Term ≥ 42+0 weeks Significant placental insufficiency risk. Induction mandated.
Abbreviations: AC (Abdominal Circumference) · ACOG (American College of Obstetricians and Gynaecologists) · AFI (Amniotic Fluid Index) · ANC (Antenatal Care) · Anti-D (Anti-D Immunoglobulin) · BPD (Biparietal Diameter) · CDC (Centers for Disease Control and Prevention) · CRL (Crown-Rump Length) · CVS (Chorionic Villus Sampling) · DIPSI (Diabetes in Pregnancy Study Group India) · DNA (Deoxyribonucleic Acid) · EDD (Estimated Date of Delivery) · EFW (Estimated Foetal Weight) · FA (Folic Acid) · FGR (Foetal Growth Restriction) · FL (Femur Length) · FMF (Foetal Medicine Foundation) · FOGSI (Federation of Obstetric and Gynaecological Societies of India) · GBS (Group B Streptococcus) · GDM (Gestational Diabetes Mellitus) · Hb (Haemoglobin) · HC (Head Circumference) · IADPSG (International Association of Diabetes and Pregnancy Study Groups) · IAP (Intrapartum Antibiotic Prophylaxis) · IFA (Iron Folic Acid) · IM (Intramuscular) · LMP (Last Menstrual Period) · MoHFW (Ministry of Health and Family Welfare) · NICU (Neonatal Intensive Care Unit) · NIPT (Non-Invasive Prenatal Testing) · NT (Nuchal Translucency) · NTD (Neural Tube Defect) · OGTT (Oral Glucose Tolerance Test) · PC-PNDT (Pre-Conception and Pre-Natal Diagnostic Techniques) · Rh (Rhesus Factor) · SBAR (Situation, Background, Assessment, Recommendation) · Td (Tetanus and adult Diphtheria) · TIFFA (Targeted Imaging for Foetal Anomalies) · TORCH (Toxoplasmosis, Other Agents, Rubella, Cytomegalovirus, Herpes Simplex) · TT (Tetanus Toxoid) · WHO (World Health Organization)
Algorithm References & Evidence Base
  1. ACOG Committee Opinion No. 700. Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
  2. ACOG Practice Bulletin No. 200. Early Pregnancy Loss. Obstet Gynecol. 2018;132(5):e197-e207.
  3. ACOG Committee Opinion No. 579. Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139-1140. Reaffirmed 2021.
  4. Ministry of Health and Family Welfare (MoHFW), Government of India. Guidelines for Antenatal Care and Skilled Attendance at Birth by ANMs/LHVs/SNs. New Delhi; 2010.
  5. FOGSI Good Clinical Practice Recommendations. Routine Antenatal Care for the Healthy Pregnant Women. The Federation of Obstetric and Gynaecological Societies of India; 2024.
  6. Government of India. Pre-Conception and Pre-Natal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994.
  7. Seshiah V, et al. DIPSI Guidelines: Diagnosis and Management of Gestational Diabetes Mellitus. J Assoc Physicians India. 2006;54:622-628.
  8. MoHFW, Government of India. Technical and Operational Guidelines for Hyperglycaemia in Pregnancy. New Delhi.
  9. World Health Organization. WHO Recommendations on Antenatal Care for a Positive Pregnancy Experience. Geneva: WHO; 2016.
How to Cite This Tool

AMA Style:
Umakanth S. Pregnancy Dating & Antenatal Scheduler. MEDiscuss. Published 2026. Accessed .

Vancouver Style:
Umakanth S. Pregnancy Dating & Antenatal Scheduler [Internet]. MEDiscuss.org; 2026 [cited ]. Available from:

Disclaimer and scope
Disclaimer: This decision-support system is intended for healthcare professionals. It does not substitute clinical judgment. These algorithms are evidence-based, but medical knowledge evolves continuously. The treating physician retains absolute responsibility for all diagnostic, dosing, and therapeutic decisions. Always verify outputs against current institutional protocols and individual patient contexts.
© 2026 MEDiscuss.org. All rights reserved. This CDSS Module was designed and developed by Dr. Shashikiran Umakanth using the available evidence base. Provided free of charge for clinical use. Unauthorised reproduction or redistribution is strictly prohibited.
Category Foundational CalculatorsScheduler
Specialties Obstetrics & Gynaecology
Status Essential
Written and maintained by Dr Shashikiran Umakanth.
Last revised: 29 July 2026
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