How Long Is Pregnancy? Weeks, Months, Trimesters and Full Term

• A full pregnancy is about 40 weeks, or roughly 9 calendar months and 1 week, counted from the first day of your last menstrual period (LMP) — not from conception.
• Pregnancy is split into three trimesters of about 13 weeks each, with different body changes and antenatal checks in each.
• Your due date (EDD) is calculated by Naegele's rule (LMP plus 280 days) and confirmed by an early dating ultrasound, which is the most accurate method.
• Only about 4 percent of babies arrive on the exact due date; the normal range for birth is 37 to 42 weeks.
• ACOG and FOGSI define 39+0 to 40+6 weeks as full term; induction is usually offered around 41+0 to 41+6 weeks if labour has not begun.
• Weeks are the precise measure your obstetric team uses — months are for family conversations.

In modern obstetrics, pregnancy is dated from the first day of your last menstrual period (LMP). This convention is used worldwide by ACOG, RCOG, NICE, WHO and FOGSI, and it exists for a simple, practical reason: most women can recall when their last period started, whereas the exact day of conception is impossible to observe and can happen anywhere within a few days around What Ovulation Actually Means.

The trade-off is that the first two 'weeks' of pregnancy are weeks when you are not actually pregnant yet. In week 1 you are menstruating; around the end of week 2 you ovulate. For a typical 28-day cycle, conception happens around day 14, which falls in week 2 of the LMP count. The fertilised egg then takes about 6 to 12 days to implant — Implantation Bleeding vs Early Period: How to Tell Them Apart usually lands in week 3 to early week 4.

By week 4, the embryo has implanted and is producing enough human chorionic gonadotropin (hCG) to turn a home test positive. So a woman with a 28-day cycle who tests on the day her period is due is already about 4 weeks pregnant by LMP dating, even though she conceived only about 2 weeks earlier.

This is why total pregnancy adds up to 280 days from LMP — that is 40 weeks, or roughly 9 calendar months and 1 week. 'Conception age' (also called fertilisation or embryonic age) is a separate measure used mainly in embryology and IVF, and it runs about 2 weeks behind gestational age. Almost every pregnancy week-by-week guide you read, including ours on what to expect week by week, uses LMP-based gestational age unless it says otherwise.

Pregnancy is divided into three trimesters of roughly 13 weeks each. Each phase has its own body changes, common symptoms and clinical priorities.

• The first antenatal booking visit, ideally before 12 weeks (commonly 8 to 10 weeks)
• An early dating ultrasound, usually between 7 and 13+6 weeks
• Baseline blood tests: complete blood count, blood group and Rh typing, HIV, syphilis, hepatitis B, thyroid function, fasting blood sugar and urine routine
• Folic acid and iron supplementation
• First-trimester screening for chromosomal conditions (the combined test or NIPT) if you choose it

Often called the 'honeymoon' trimester: nausea usually settles, energy returns, and your bump shows without being uncomfortable. Most women first feel fetal movements (quickening) between 18 and 22 weeks.

This is when the detailed anomaly scan (TIFFA or level 2 scan) is performed, between 18 and 22 weeks, to assess your baby's anatomy. Screening for gestational diabetes with the DIPSI or glucose tolerance test is done at 24 to 28 weeks under FOGSI protocols. If you are Rh-negative, this is also when planning for anti-D immunoglobulin begins.

Body changes intensify: weight gain accelerates, back and pelvic discomfort emerge, sleep gets harder, Braxton Hicks contractions become noticeable, and breathlessness sets in as the uterus crowds the diaphragm. Antenatal visits become more frequent — every 2 to 4 weeks until 36 weeks, then weekly.

The Tdap vaccine is given at 27 to 36 weeks. Growth scans, fetal position checks and birth planning all step up. If your baby is breech near term, your doctor may discuss external cephalic version (ECV) at 36 to 37 weeks. The trimester ends with labour and delivery, ideally in the full-term window of 39+0 to 40+6 weeks, with a normal range of 37 to 42 weeks.

Your estimated due date (EDD), or expected date of delivery, is first worked out using Naegele's rule, named after the 19th-century obstetrician Franz Naegele. The rule: EDD equals the first day of your last period plus 280 days — the easy mental shortcut is to subtract 3 months and add 7 days (adjusting the year).

For example, if your LMP was 1 January 2026: subtract 3 months (1 October 2025), add 7 days (8 October 2025), add a year — EDD is 8 October 2026. Indian obstetricians use this at the first antenatal visit.

Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter or irregular — for instance with PCOS, recent contraceptive use or breastfeeding — the LMP date is less reliable, and your doctor will lean on the dating ultrasound instead.

The first-trimester dating ultrasound is the single most accurate way to date a pregnancy. ACOG, RCOG and FOGSI all treat crown-rump length (CRL) measured between 7+0 and 13+6 weeks as the gold standard, accurate to within about 5 to 7 days when done between 8 and 12 weeks. After 14 weeks, dating accuracy drops as babies naturally vary more in size.

If LMP-based dating and ultrasound dating differ by more than 7 days in the first trimester (or more than 10 to 14 days later in the second trimester), the EDD is reassigned based on the ultrasound. Women who conceive through IVF have the most precise dating of all, because the embryo transfer date is known exactly. Once your EDD is set, it is rarely changed later in pregnancy — and every decision about screening, growth and delivery timing is counted from it.

• Early term: 37+0 to 38+6 weeks
• Full term: 39+0 to 40+6 weeks
• Late term: 41+0 to 41+6 weeks
• Post term: 42+0 weeks and beyond

Outcomes are best when delivery happens in the full-term window. Babies born at early term, though not preterm, have slightly higher rates of breathing difficulty, NICU admission, feeding problems and jaundice. Because of this, elective deliveries — a planned cesarean or induction of labour without a medical reason — should not be scheduled before 39+0 weeks, a principle endorsed by ACOG, RCOG, NICE and FOGSI. When a cesarean is medically needed, it is usually scheduled at 39+0 weeks to balance maturity against the chance of labour starting on its own.

Late-term and post-term pregnancy carry a modest rise in risks: stillbirth (which increases slowly from 39 weeks and more notably after 41), meconium aspiration, a larger baby (macrosomia) with shoulder dystocia risk, and low amniotic fluid. For this reason, FOGSI and international guidelines recommend offering induction at 41+0 to 41+6 weeks if labour has not started, supported by evidence showing fewer perinatal deaths and fewer cesareans than waiting. By 42 weeks, induction is strongly recommended unless there is a specific reason not to.

At the other end, preterm birth means delivery before 37+0 weeks. India's preterm birth rate is roughly 13 percent — among the highest in the world — and is a major contributor to newborn deaths. Late preterm birth (34 to under 37 weeks) is the most common category and is often linked to conditions like preeclampsia, growth restriction or early rupture of membranes.

• First pregnancies tend to run slightly longer than later ones (median around 40+5 versus 40+3 weeks)
• Genetics matter — if your own mother went post-term, you are more likely to as well
• Twins and multiples deliver earlier, often around 36 to 37 weeks
• Certain conditions prompt earlier delivery: gestational diabetes, preeclampsia, fetal growth restriction, placenta previa or cholestasis

• Month 1: weeks 1 to 4
• Month 2: weeks 5 to 8
• Month 3: weeks 9 to 13
• Month 4: weeks 14 to 17
• Month 5: weeks 18 to 22
• Month 6: weeks 23 to 27
• Month 7: weeks 28 to 31
• Month 8: weeks 32 to 35
• Month 9: weeks 36 to 40

By this convention, the first trimester is roughly months 1 to 3, the second is months 4 to 6, and the third is months 7 to 9, with completion of month 9 landing at week 40 — your EDD.

Some of these months map onto Indian traditions. The seemandham (a South Indian baby shower) is usually held in the seventh month, around weeks 28 to 31, and the godh bharai (North Indian baby shower) in the seventh or eighth month. Many families also expect the mother to return to her parental home (maika) for the last month or two before delivery, often around weeks 32 to 36. These are lovely customs — just coordinate them with your medical care, especially if travelling to your maika means moving to a different city where antenatal care has to be re-established, and arrange any pregnancy travel with your doctor.

The practical rule: use months with family, but use weeks with your obstetric team. Weeks are the precise measure every clinical decision is based on.

• A non-stress test (NST) or cardiotocography (CTG) to check the baby's heart rate pattern
• An amniotic fluid index (AFI) measurement
• A discussion of induction options
• Sometimes a membrane sweep ('stretch and sweep') from 40 weeks, which has modest evidence of helping labour start

Induction at 41+0 to 41+6 weeks is the standard recommendation in Indian obstetric practice. Methods include cervical ripening (prostaglandin gel, vaginal pessary or a balloon catheter), artificial rupture of the membranes (amniotomy) and an oxytocin drip, chosen based on your cervix (the Bishop score) and history. You can read more in our guide to the induction of labour process and options and the stages of labour.

Waiting beyond 41+6 weeks is uncommon now. If you strongly prefer to wait, that calls for twice-weekly NST and fluid checks and a frank conversation about stillbirth, meconium and large-baby risks. Some families also prefer 'auspicious' delivery dates, which can conflict with medical timing — the goal is shared decision-making that respects your values while keeping your baby safe.

• Booking visit and dating scan: before 12 weeks (ideally 8 to 10 weeks)
• NT scan with the double-marker test for first-trimester screening: 11 to 13+6 weeks; NIPT can be offered from 10 weeks
• Anomaly scan (TIFFA): 18 to 22 weeks
• Gestational diabetes test (DIPSI or OGTT): 24 to 28 weeks
• Anti-D for Rh-negative mothers: around 28 weeks
• Tdap vaccine: 27 to 36 weeks
• Growth scans: 28 to 32 weeks, with extra scans at 36 to 38 weeks if needed
• Fetal position checks and final birth planning: from 36 weeks

By 36 weeks, aim to have your hospital bag packed, your birth plan discussed with your obstetrician and your transport to the hospital confirmed. From 37 weeks you are 'term', and labour can begin any time — your doctor will go over the signs of labour and when to come in. Following this schedule maximises the chance of catching complications early and supports a good outcome for you and your baby. For a deeper week-by-week walkthrough, see our full guide on what to expect week by week.

• Vaginal bleeding, or a gush or steady trickle of fluid (possible rupture of membranes)
• Regular, painful contractions before 37 weeks (possible preterm labour)
• A noticeable drop in your baby's movements, especially after 28 weeks
• Severe or persistent headache, blurred vision, or sudden swelling of the face and hands (possible signs of preeclampsia)
• Severe abdominal pain, high fever, or fainting
• Reaching 41 weeks without labour starting — to plan monitoring and induction

• ACOG & SMFM — Definition of Term Pregnancy (Committee Opinion)
• ACOG — Methods for Estimating the Due Date (Committee Opinion 700)
• WHO — Recommendations on Antenatal Care for a Positive Pregnancy Experience
• NICE — Inducing Labour (NG207)
• FOGSI — Good Clinical Practice Recommendations