How many weeks pregnant you are, your trimester, and baby's size this week
| Stage | Weeks | Days from LMP | What defines it |
|---|---|---|---|
| First trimester | 1 – 13 | 0 – 97 | Conception, implantation, organ formation; highest miscarriage risk |
| Second trimester | 14 – 27 | 98 – 195 | Energy returns, anatomy scan at 18–22 weeks, first kicks |
| Third trimester | 28 – 40 | 196 – 280 | Rapid growth, kick counts, weekly appointments from 36 |
| Early term (ACOG) | 37w0d – 38w6d | 259 – 272 | Born safely in most cases, but brain and lungs still maturing |
| Full term | 39w0d – 40w6d | 273 – 286 | The ideal delivery window |
| Late term | 41w0d – 41w6d | 287 – 293 | Monitoring intensifies |
| Postterm | 42w+ | 294+ | Induction usually recommended |
| Week | Milestone | Average size |
|---|---|---|
| 4 | Implantation complete; positive pregnancy test | 0.04 in, poppy seed |
| 6 | Heartbeat visible on ultrasound (~110 bpm) | 0.13 in, lentil |
| 10 | Graduates from embryo to fetus; organs in place | 1.2 in, 0.14 oz, prune |
| 12 | Nuchal translucency scan window closes; reflexes begin | 2.1 in, 0.5 oz, lime |
| 16 | Hearing begins; facial expressions | 4.6 in, 3.5 oz, avocado |
| 20 | Halfway; anatomy scan | ~10 in, 10.6 oz, banana |
| 24 | Viability milestone; lungs make surfactant | 11.8 in, 1.3 lb, ear of corn |
| 28 | Third trimester; kick counting starts | 14.8 in, 2.2 lb, eggplant |
| 32 | Lanugo falls out; practice breathing strong | 16.7 in, 3.7 lb, jicama |
| 37 | Early term | 19.1 in, 6.3 lb, bunch of kale |
| 39 | Full term | 20.0 in, 7.3 lb, mini watermelon |
| 40 | Due week (only ~4–5% arrive on the due date itself) | 20.2 in, 7.6 lb, small pumpkin |
Sizes are commonly published averages from fetal growth charts (crown-rump length before week 20, crown-heel after). Normal variation at every week is wide; your ultrasound's measurements always outrank a chart.
"How many weeks pregnant am I?" has a counterintuitive answer: the count starts before conception. Gestational age is measured from the LMP because that date is identifiable while ovulation usually isn't. Weeks 1 and 2 are, strictly, not pregnancy at all; conception typically happens at the start of week 3.
Gestational age = days since LMP, expressed as weeks (floor of days ÷ 7) plus remaining days. You're "in week N" where N = floor(days ÷ 7) + 1, capped at 40 for display purposes. Due date = LMP + 280 days. Trimesters break at day 98 (14w0d) and day 196 (28w0d). If you start from a due date, LMP = due − 280; from conception, LMP = conception − 14.
Pick your starting point. Most people know their LMP; if you have an IVF or ultrasound due date, use the due-date mode, which is more accurate than a remembered period. The result shows your exact gestational age, where you are in the 40-week arc, and a link to this week's detailed page — size comparison, development milestone, symptoms, and what to do next.
LMP June 1, 2026. On August 26, that's 86 days of gestation: 12 weeks 2 days, in week 13 of 40, still first trimester (the second begins at day 98, 14w0d). Due date = June 1 + 280 days = March 8, 2027, 194 days away, and the pregnancy is 86 ÷ 280 = 31% along. At week 13 the baby averages 2.9 inches and 0.8 oz, roughly a peach, with fingerprints beginning to form on fingertips the size of a grain of rice.
Cycle length matters: Naegele's rule assumes 28 days with ovulation on day 14. On a 34-day cycle you likely ovulated about six days later than the rule assumes, so an early ultrasound's dating supersedes both. Due dates are estimates, and only about 4 to 5 in 100 babies arrive on theirs.
Count the days from the first day of your last menstrual period (LMP) to today, then divide: 86 days is 12 weeks and 2 days, putting you in week 13 of 40. Doctors count from the LMP, not conception, because the LMP date is usually known while ovulation isn't. That's also why weeks 1 and 2 happen before you're technically pregnant.
Naegele's rule: due date = LMP + 280 days (40 weeks), assuming a 28-day cycle with ovulation on day 14. An LMP of June 1 gives a due date of March 8 the next year. Longer or shorter cycles shift ovulation, so a first-trimester ultrasound is the more accurate dating method; it can move your due date by days to more than a week.
The first trimester runs from week 1 through the end of week 13 (days 0–97 from LMP). The second trimester covers weeks 14–27. The third trimester runs from week 28 to birth, usually week 40. ACOG also divides the end into early term (37–38 weeks), full term (39–40), late term (41), and postterm (42+).
Average size climbs from a poppy seed at week 4, to a lime at week 12 (2.1 inches, 0.5 oz), a banana at the week-20 halfway point (about 10 inches crown to heel, 10.6 oz), an ear of corn at week 24 (1.3 lb), and a small pumpkin at week 40 (roughly 20 inches, 7.6 lb). These are averages from fetal growth charts; normal variation at every week is wide.
Around 24 weeks is the conventional viability milestone, when survival with intensive neonatal care becomes possible and rises steeply with each added week. Week 24 is also when lungs begin producing surfactant. By 28 weeks survival odds in a modern NICU are high; by 37 weeks a baby is considered early term and usually needs no NICU time.
From the last period. Gestational age is counted from the LMP because it's a date most people can pinpoint, while conception is invisible and typically happens about two weeks later. If you know your exact conception date (from IVF or ovulation tracking), convert by adding 14 days to get the equivalent LMP-based age.