1📐 What is Gestational Age? How to Calculate it from LMP, Ultrasound & CRL
Gestational age is the clinical measure of how far a pregnancy has progressed, always counted from the first day of the Last Menstrual Period (LMP) — not from conception. It is expressed in completed weeks plus days (e.g., 12 weeks 3 days, written as 12+3 or 12w3d). A full-term pregnancy is 40 weeks 0 days gestational age (280 days from LMP).
How to Calculate Gestational Age from LMP
- Note Day 1 of your LMP — the universal starting point used by all obstetricians worldwide (ACOG, FOGSI, WHO)
- Count total days from LMP to today — for example, 77 days from LMP
- Divide by 7 — 77 ÷ 7 = 11 weeks 0 days. You are 11+0 gestational age
- Confirm with CRL ultrasound — if the CRL-based gestational age differs by more than 7 days, use the ultrasound value as the definitive gestational age
Gestational Age from Ultrasound Biometry (CRL, BPD)
Ultrasound biometry calculates gestational age from physical measurements of the fetus. Different measurements apply to different stages:
| Measurement | Gestational Age Range | Formula | Accuracy |
|---|---|---|---|
| CRL — Crown-Rump Length 10–84 mm |
6+0 to 13+6 weeks | GA (wks) = 5.2876 + (0.1584 × CRL) − (0.0007 × CRL²) Hadlock 1992; ISUOG gold standard |
±3–5 days |
| BPD — Biparietal Diameter 17–97 mm |
13 to 40 weeks | GA (wks) = (2 × BPD in mm + 44.2) / 7 Hadlock formula; combined with HC preferred |
±1–2 weeks (2nd tri) |
| HC — Head Circumference | 13 to 40 weeks | Used with BPD for composite gestational age. Preferred over BPD alone beyond 28 weeks | ±1–2 weeks |
| FL — Femur Length | 14 to 40 weeks | Used in composite with BPD and HC. Useful when head measurements are difficult | ±2–3 weeks (3rd tri) |
Gestational Age vs Fetal Age — What's the Difference?
Gestational age (GA) is always 2 weeks more than fetal age (also called embryonic age or conceptional age). Doctors use gestational age universally in scan reports, prescription notes, and hospital records. When a radiologist writes "GA 12+3 weeks" in a scan report, it means 12 weeks and 3 days from the LMP, not from fertilisation.
2LMP vs Ultrasound vs CRL vs IVF: Which EDD Method is Most Accurate?
Indian obstetricians and radiologists use up to five methods to calculate EDD and gestational age. Accuracy varies significantly — understanding which method to trust avoids unnecessary interventions.
| Method | Accuracy | Best Used When | FOGSI / ACOG Recommendation |
|---|---|---|---|
| CRL Ultrasound 7–13+6 weeks, 10–84 mm |
±3–5 days | Always preferred in first trimester — gold standard | Gold standard. If >7 days difference from LMP, use CRL-based date. |
| IVF Transfer Date Known fertilisation |
±1–2 days | IVF / FET pregnancies — exact transfer date known | Most precise method. Transfer date + (266 − embryo age). |
| LMP (Naegele's Rule) 21–45 day cycle adjustment |
±2 weeks | Regular cycles, first-trimester scan not yet done | Starting point only. Confirm with 8–10 week dating scan. |
| BPD / HC / FL Biometry After 14 weeks |
±1–2 weeks (2nd tri) | When first-trimester scan was not performed | Acceptable. Use composite of BPD + HC + FL; less accurate alone. |
| Second-trimester Ultrasound After 14 weeks |
±2–3 weeks | When first-trimester scan was not done | Acceptable if no earlier scan. Avoid changing EDD if <14 day discrepancy. |
Key Clinical Numbers to Know During Pregnancy
3🔬 Indian Antenatal Scan Schedule by Gestational Age (FOGSI 2024)
| Gestational Age | Scan / Test | Purpose |
|---|---|---|
| 6–8 weeks GA | Early viability scan | Confirm intrauterine pregnancy, heartbeat, rule out ectopic. Not routine if asymptomatic |
| 8–10 weeks GA | Dating scan (CRL) | Confirm gestational age by CRL. Correct EDD if differs >7 days from LMP |
| 11–13+6 weeks GA | NT scan + Dual Marker Recommended | Nuchal translucency for Down syndrome risk (trisomy 21, 18, 13). Combined with PAPP-A and β-hCG |
| 18–20 weeks GA | Anomaly scan (Level II / TIFFA) Most Important | Detailed fetal anatomy — heart, brain, spine, limbs, kidneys, abdominal wall. Detects >80% of structural anomalies |
| 24–28 weeks GA | 75g OGTT — GDM screening India | DIPSI protocol: 75g glucose non-fasting, 2h blood sugar ≥140 mg/dL = GDM. Indian women have 16–17% GDM prevalence |
| 28 weeks GA | Anti-D injection | For Rh-negative mothers. Prevents Rh incompatibility and haemolytic disease in subsequent pregnancies |
| 30–32 weeks GA | Growth scan + Colour Doppler | Estimate fetal weight, amniotic fluid index, placental position. Uterine and umbilical artery Doppler for IUGR screening |
| 36 weeks GA | Group B Streptococcus (GBS) swab | Recommended at tertiary centres. If positive, IV antibiotics in labour prevent neonatal GBS sepsis |
| 36–40 weeks GA | Final growth scan / CTG | Fetal wellbeing, presentation (vertex/breech), liquor volume, decide delivery plan |
4🏛️ Government Schemes for Pregnant Women in India
All pregnant women in India are entitled to free antenatal care and financial support through central government schemes. Register early — most schemes require enrolment within the first trimester.
Documents needed: Aadhaar card, MCP (Mother & Child Protection) card, BPL card (if applicable), bank account details. Register through your nearest Anganwadi centre or ANM at the primary health sub-centre.
5🥗 Indian Pregnancy Nutrition Guide — Trimester by Trimester
First Trimester — Gestational Age 1 to 13 Weeks
- Folic acid 5 mg/day: Neural tube defect prevention — ideally started before conception and continued until 12 weeks gestational age. Available free at government hospitals
- Iron + Folic Acid (IFA) tablet: ICMR recommends 180 mg elemental iron + 500 µg folic acid daily from first trimester to prevent anaemia, which affects 50%+ of Indian pregnant women
- Nausea management: Ginger tea, jeera (cumin) water, and small frequent meals are safe. Avoid raw papaya, large amounts of pineapple, and sesame seeds which may stimulate uterine contractions
Second Trimester — Gestational Age 14 to 27 Weeks
- Calcium 1,000–1,200 mg/day: Essential for fetal bone and tooth formation. Sources: milk, curd, paneer, ragi (finger millet), til (sesame). Calcium carbonate supplements widely prescribed in India
- Protein 70–100 g/day: Dal, rajma, chana, eggs, milk, paneer, soya. Most Indian women are protein-deficient in pregnancy — prioritise at every meal
- GDM screening at 24–28 weeks gestational age: Indian women have 4× higher GDM risk. DIPSI 75g OGTT (non-fasting). If positive: low-glycaemic diet, walking 30 minutes daily, insulin if targets not met
Third Trimester — Gestational Age 28 to 40 Weeks
- Iron absorption tips: Take iron tablets at bedtime (not with milk or tea). Vitamin C (amla, lemon juice) doubles absorption. Dark stools are a normal side effect of iron tablets
- Omega-3 (DHA): Walnuts, flaxseeds, chia seeds, fatty fish (safe 2×/week). Important for fetal brain and retinal development. Many obstetricians prescribe DHA 200–300 mg/day from second trimester
- Absolute restrictions: Alcohol (zero safe amount), raw or undercooked meat, unpasteurised milk, excess Vitamin A (liver, cod liver oil), caffeine >200 mg/day (limit to under 2 cups of tea)
6🚑 Signs of Labour — When to Go to Hospital
True Labour Signs (after 37 Weeks Gestational Age)
- Regular contractions — the 5-1-1 rule: Contractions every 5 minutes, lasting 60 seconds each, for at least 1 hour. Starting in the back and radiating to the front, progressively stronger and longer
- Rupture of membranes (PROM): Sudden gush or continuous trickle of clear/pale yellow fluid. Go to hospital within 1–2 hours even without contractions — risk of cord prolapse and infection increases after 18 hours
- Show (bloody mucous plug): Passage of blood-stained mucus signals that the cervix is beginning to dilate. Labour usually starts within 24–48 hours. Not an emergency, but inform your obstetrician
- Reduced fetal movements: Fewer than 10 movements in 2 hours at any point in the third trimester — go to hospital immediately for CTG (cardiotocography) monitoring
Kick Count — Self-Monitoring After 28 Weeks Gestational Age
Count fetal movements daily — aim for 10 movements within 2 hours after a meal, when the baby is typically most active. Keep a kick count diary in your MCP card. This is the most important self-monitoring tool in the third trimester and has been shown to detect fetal distress early.
7Frequently asked questions
What is a due date calculator and how does it work?
A due date calculator (also called an EDD calculator) estimates your Estimated Date of Delivery by adding 280 days (40 weeks) to the first day of your Last Menstrual Period, or by using an ultrasound measurement, IVF transfer date, or conception date instead. This calculator supports all five methods — LMP, ultrasound, CRL/BPD biometry, IVF/FET, and conception date — and shows your current gestational age alongside the EDD.
How accurate is an online due date calculator?
A due date calculator based on LMP alone is accurate to within about 2 weeks, since it assumes a standard 28-day cycle and known ovulation timing. Accuracy improves significantly with a first-trimester CRL ultrasound (±3–5 days) or a known IVF transfer date (±1–2 days). Only about 5% of babies are born exactly on their calculated due date; most deliver within 1–2 weeks either side of the EDD.
What is gestational age and how is it calculated?
Gestational age is the clinical measure of how far a pregnancy has progressed, counted from the first day of the Last Menstrual Period (LMP) — not from the date of conception. It is expressed in completed weeks plus days (e.g., 12 weeks 3 days, written as 12+3). To calculate gestational age: subtract the LMP date from today's date to get total days, then divide by 7. A full-term pregnancy is 40 weeks (280 days) gestational age.
How do I calculate gestational age from CRL on ultrasound?
Gestational age from CRL (Crown-Rump Length) is calculated using the Hadlock formula: Gestational Age (weeks) = 5.2876 + (0.1584 × CRL in mm) − (0.0007 × CRL²). This formula applies from 6 to 13 weeks 6 days (CRL up to approximately 84 mm). CRL-based gestational age is accurate to ±3–5 days and is the gold standard for pregnancy dating per ISUOG, ACOG, and FOGSI guidelines. Beyond 84 mm CRL, use BPD or HC measurements.
How is the due date calculated from LMP?
Using Naegele's Rule: add 280 days (40 weeks) to the first day of the Last Menstrual Period (LMP), adjusted for cycle length. For a 28-day cycle this equals LMP + 9 months + 7 days. Our calculator automatically adjusts for cycle lengths of 21 to 45 days — longer cycles push the EDD later, shorter cycles bring it forward.
How many weeks pregnant am I? How to find out gestational age today
To find out how many weeks pregnant you are today: (1) Note the first day of your Last Menstrual Period. (2) Count the number of days from that date to today. (3) Divide by 7 and express as weeks + remaining days. For example, if 77 days have passed since your LMP, you are 11 weeks 0 days pregnant (gestational age 11+0). Use this calculator's LMP tab to get your gestational age instantly.
How accurate is ultrasound dating compared to LMP for due date?
An early ultrasound measuring Crown-Rump Length (CRL) at 7–13 weeks is accurate to ±5–7 days and is the clinical gold standard. LMP dating is accurate to ±2 weeks. If the ultrasound EDD differs from LMP EDD by more than 7 days, FOGSI and WHO guidelines recommend using the ultrasound-based date as the official EDD.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the LMP and is the universal clinical standard — it adds approximately 2 weeks before actual fertilisation occurred. Fetal age (embryonic age or conceptional age) is counted from the date of fertilisation (conception) and is always approximately 2 weeks less than gestational age. For example, at 12 weeks gestational age, the fetal age is approximately 10 weeks. Doctors always use gestational age in clinical records, scan reports, and antenatal care.
What is the due date formula for IVF Day 5 blastocyst transfer?
For a Day 5 blastocyst transfer: EDD = Transfer Date + 261 days. For Day 3 embryo transfer: EDD = Transfer Date + 263 days. The general formula is EDD = Transfer Date + (266 minus embryo age in days at transfer), since all pregnancies are 266 days from fertilisation.
When should I get my first antenatal check-up in India?
Ideally within 8–10 weeks of pregnancy. Your first ANC visit includes blood tests (CBC, blood group and Rh, blood sugar, TSH, VDRL, HIV, Hepatitis B), urine examination, blood pressure, weight, and a dating ultrasound scan. Register for your MCP (Mother and Child Protection) card and government schemes like PMMVY and Janani Suraksha Yojana at your nearest PHC or sub-centre.
What is the DIPSI protocol for gestational diabetes screening in India?
DIPSI (Diabetic Pregnancy Study Group India): Administer 75g oral glucose at any time of day without fasting. Measure blood sugar at 2 hours. A result of 140 mg/dL or above is diagnostic for GDM. The DIPSI method is preferred in India because Indian women have a GDM prevalence of 16–17%, approximately 4 times higher than Western populations.
Can my due date change after an early ultrasound?
Yes. If the first-trimester ultrasound EDD differs from the LMP-based EDD by more than 7 days, the EDD is revised to the ultrasound date. Multiple revisions during pregnancy are not recommended — the first-trimester CRL scan is the most accurate single measurement and should be used as the final EDD.
Is it safe to travel during pregnancy in India?
The safest period to travel is the second trimester, from 14 to 28 weeks. Avoid long-distance travel after 36 weeks. Always carry your MCP card and emergency contact details. On flights, walk every 2 hours to prevent DVT, wear compression stockings, and stay hydrated. Most Indian and international airlines allow travel up to 36 weeks with a medical certificate from your obstetrician.
Which vaccines are recommended during pregnancy in India?
Tetanus toxoid (Td): 2 doses in the first pregnancy given at 16–24 weeks and 4 weeks apart, or 1 booster in subsequent pregnancies. Influenza vaccine is recommended. COVID-19 vaccine is recommended per current National Health Programme guidelines. All vaccines are available free of charge at government antenatal clinics.
What does gestational age at 40 weeks mean?
40 weeks gestational age means the pregnancy has reached full term — this is the Estimated Due Date (EDD). It equals 280 days from the LMP. At 40 weeks gestational age, the fetal age is approximately 38 weeks from fertilisation. Births between 37+0 and 41+6 weeks gestational age are considered term. Delivery before 37 weeks is preterm; after 42 weeks is post-term (post-dates).
Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.