Key takeaways

  • Gestational age is counted from the first day of your last menstrual period (LMP), so by convention the first 2 weeks happen before you actually conceive.
  • A full-term pregnancy is 40 weeks (280 days) from the LMP — roughly 9 calendar months plus 1 week.
  • Naegele's rule gives a first estimated due date (EDD): LMP minus 3 months, plus 7 days.
  • A first-trimester dating scan (crown-rump length, ideally 8–12 weeks) is the most accurate way to confirm or correct your dates — accurate to about ± 5–7 days.
  • Once your EDD is set in early pregnancy, it usually does not change. Later scans show how the baby is growing, not how old it is.
  • Only about 1 in 20 babies actually arrive on the EDD; most come within 2 weeks either side.

What "gestational age" actually means

  • Gestational age = counted from the first day of your LMP (the clinical standard).
  • Written as weeks + days, e.g. 12+3 = 12 weeks 3 days.
  • Weeks 1–2 happen before conception, which is why the dating feels "2 weeks ahead."
  • A pregnancy is 40 weeks (280 days) from the LMP.

Naegele's rule: how your due date is first calculated

When the LMP date is not reliable

LMP dating works well when periods are regular and remembered clearly. It becomes shaky when:


In all of these situations, a dating ultrasound becomes the main way to fix your gestational age.

The dating scan: the most accurate way to confirm your weeks

When the scan replaces your LMP date

Your doctor does not change your dates on a whim — there are clear rules. The EDD is reassigned from the scan only when the difference is bigger than expected for that stage of pregnancy:

  • Scan at 8–13+6 weeks: reassign if the scan and LMP differ by more than 7 days.
  • Scan at 14–15+6 weeks: reassign if difference is more than 7 days.
  • Scan at 16–21+6 weeks: reassign if more than 10 days.
  • Scan at 22–27+6 weeks: reassign if more than 14 days.
  • Scan at 28 weeks or later: reassign if more than 21 days.

Once your EDD is set in early pregnancy, it normally stays fixed for the rest of the pregnancy. A later growth scan tells you how big the baby is, not how old it is.

Getting a dating scan in India

First-trimester scans are widely available at hospital ultrasound departments and registered radiology centres. Every facility is regulated under the PCPNDT Act (which forbids sex determination), so reports follow a standard format. Costs range roughly from ₹50–300 in the public sector to ₹800–2,500 privately. Beyond dating, the early scan also confirms the pregnancy is inside the uterus (ruling out an ectopic pregnancy), checks the heartbeat, and looks for twins or more.

Gestational age vs fetal age: why they differ by about 2 weeks

How dating works after IVF

Pregnancies from IVF have the most precise dates of all, because the day of fertilisation is known exactly. Gestational age is calculated from the embryo's age plus the conventional 2 weeks. For a day-5 blastocyst, the EDD is the transfer date plus 266 days (fresh) or plus 263 days (frozen). For IUI, dating runs from the insemination date, which matches ovulation. These calculations are accurate to within a day or two, so IVF dates are rarely changed by a scan.

Special situations that change how dating is done

Some pregnancies do not fit the neat LMP-plus-scan model. In these cases, a first-trimester ultrasound usually becomes the primary way to set your dates:

  • Irregular cycles — PCOS, thyroid problems, recent weight change or stress can shift ovulation, so the LMP is documented but treated as approximate.
  • No periods before pregnancy — breastfeeding (lactational amenorrhoea), hormonal contraceptives, hormonal IUDs, implants or injections may mean you never had an LMP to count from.
  • Pregnant soon after stopping contraception — the first cycle off the pill, injection or hormonal IUD may not be regular.
  • Conceiving before your first period returns — after a delivery or a Miscarriage: Types, Recovery and Care in India, ovulation can come before your period does, so there is no reliable LMP.
  • Twins and multiples — the larger baby's CRL is used for dating (the international standard); twin pregnancies tend to deliver earlier, around 36–37 weeks.

The shared principle is simple: when the LMP cannot be trusted, the earliest available scan sets the dates, and your antenatal record documents which method was used and why.

What each gestational age means for your antenatal care in India

  • Weeks 4–6: First positive test; continue or start folic acid and prenatal vitamins.
  • Weeks 7–12: First antenatal booking visit (ideally before 12 weeks per FOGSI) — history, examination, blood group and Rh, CBC, HIV, syphilis, hepatitis B, thyroid, blood sugar, urine, and early dating scan.
  • Weeks 10–13+6: NIPT (from 10 weeks if chosen) and first-trimester combined screening — the NT scan with PAPP-A and free beta-hCG.
  • Weeks 15–20: Quadruple marker test if first-trimester screening or NIPT was not done.
  • Weeks 18–22: Anomaly scan (TIFFA / level-2 scan) for detailed fetal anatomy — part of birth-defects screening.
  • Weeks 24–28: Gestational diabetes screening (DIPSI or 75 g OGTT) and treatment of any Anemia in Pregnancy in India: Cutoffs, IFA, Diet & Treatment.
  • Weeks 27–36: Tdap vaccine (ideally 27–32 weeks) and flu vaccine in season.
  • Week 28: Anti-D immunoglobulin for Rh-negative mothers; visits move to every 2 weeks.
  • Weeks 28–36: Third-trimester growth scans and fetal position checks; ECV offered for breech at 36–37 weeks.
  • Week 37: Early term begins; baby is considered term; visits become weekly.
  • Weeks 39–40+6: Full term — the optimal delivery window. Elective delivery without medical reason should not be before 39+0 (ACOG, FOGSI).
  • Weeks 41–41+6: Late term; induction of labour usually offered with close monitoring.
  • 42 weeks+: Post term; induction strongly recommended due to higher stillbirth risk.

Weeks vs months: talking about your pregnancy

Cultural milestones, work and travel

Indian celebrations are usually pinned to the month: a seemandham or godh bharai baby shower typically falls in the 7th–8th month (about weeks 28–35), and returning to the maika for delivery is often planned around the 8th month. Coordinating these with your scan dates is easy once you know both your weeks and the month equivalent.

For work, maternity leave under the Maternity Benefit (Amendment) Act, 2017 is 26 weeks for the first two children (12 weeks thereafter) and can begin up to 8 weeks before your EDD. For travel, most airlines restrict flights after about 36 weeks domestically and 32–36 weeks internationally — always measured in gestational weeks — so plan trips accordingly.

When to see a doctor

  • You have a positive pregnancy test and have not yet had a first-trimester dating scan — aim to be seen before 12 weeks.
  • You are unsure of your LMP, have irregular cycles or had no periods before conceiving — you need an early scan to set accurate dates.
  • You have bleeding, severe one-sided pelvic pain, dizziness or shoulder-tip pain in early pregnancy — these can signal an ectopic pregnancy and need urgent review.
  • You reach 41 weeks without labour starting — discuss monitoring and induction.
  • You feel reduced or absent baby movements at any stage from the time movements are established — contact your team the same day.
  • Your dates from LMP and your scan differ a lot, or you are confused about which due date is correct — ask your obstetrician to confirm the documented EDD.

Myths vs Facts

Frequently asked questions

Why am I "pregnant" two weeks before I conceived?

Because gestational age is counted from the first day of your last period, not from conception. Ovulation and conception usually happen around the end of week 2, so the calendar is always about 2 weeks "ahead" of the actual age of the embryo. This is the universal convention used by all obstetric societies.

Which is more accurate — my LMP or my dating scan?

A first-trimester dating scan (crown-rump length at 8–12 weeks) is more accurate, to about ± 5–7 days, while LMP dating depends on remembering your period correctly and on a regular cycle. If the two disagree by more than 7 days in the first trimester, your doctor will reassign your due date based on the scan.

Can my due date change later in pregnancy?

Usually not. Once your EDD is set early on, it stays fixed. Later scans measure how the baby is growing, not how old it is, so they are not used to re-date the pregnancy except in unusual circumstances. The earliest accurate scan is the one that counts.

How do I convert weeks to months for my family?

A rough guide: month 3 ≈ weeks 9–13, month 5 ≈ weeks 18–22, month 7 ≈ weeks 28–31, month 9 ≈ weeks 36–40. Trimesters are simpler still — first (up to 13+6 weeks), second (14–27+6) and third (28 weeks onward). For anything medical, stick to weeks.

What if I don't remember my last period or my cycles are irregular?

That is common and easily handled. Your doctor will rely on an early dating scan to set your gestational age instead of the LMP. This is the standard approach for women with irregular cycles, no periods before pregnancy, or recent contraceptive use.

Will my baby be born exactly on the due date?

Almost certainly not — only about 1 in 20 babies arrive on the exact EDD. Most are born within 2 weeks before to 2 weeks after it. The due date is a planning reference, not a deadline; full term is the window from 39 to 40+6 weeks.

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