A pregnancy is 40 gestational weeks, counted from the first day of your last period, and only about 38 of those involve an actual baby. Due date = LMP + 280 days. Trimesters split at weeks 14 and 28, viability lands near 24 weeks, and full term is 39 to 40. Here's the whole arc, with the arithmetic explained.
Gestational age starts at day one of your last menstrual period (LMP), not at conception. That sounds like a design flaw, but it's practical: the LMP is a date most people can name, while ovulation is invisible. On the standard 28-day cycle, conception happens near the start of week 3, so weeks 1 and 2 are preparation, and you're "two weeks pregnant" before any sperm meets any egg.
The arithmetic is one rule, Naegele's: due date = LMP + 280 days. An LMP of June 1 gives a due date of March 8 the next year. On August 26, that pregnancy is at day 86: 12 weeks 2 days, in week 13, first trimester, 31% of the way done. Cycle length is the catch: the rule assumes ovulation on day 14. A 34-day cycle likely ovulates about six days later, which is why a first-trimester ultrasound, accurate to within about 5 to 7 days, becomes the gold standard for dating.
| Trimester | Weeks (days from LMP) | The headline |
|---|---|---|
| First | 1–13 (days 0–97) | Conception, implantation, organ formation; nausea and exhaustion; highest miscarriage risk, which drops sharply after week 12 |
| Second | 14–27 (days 98–195) | The honeymoon: energy returns, anatomy scan at 18–22 weeks, first kicks, rapid growth |
| Third | 28–40+ (days 196+) | Half a pound a week of growth, kick counts, every-two-week then weekly appointments, birth prep |
The arc: conception in week 3, implantation and a positive test in week 4, a beating heart visible on ultrasound around week 6, all major organs laid down by week 10, when the embryo formally becomes a fetus. Reflexes arrive by week 12; fingerprints etch in week 13. The trade for all this construction is the roughest symptoms of the pregnancy: nausea that peaks around weeks 8 to 11, crushing fatigue, and tender everything. Two habits matter most now: a prenatal vitamin with folic acid (the neural tube closes by week 6, often before the test turns positive) and the first prenatal visit, usually scheduled around week 8. First-trimester screening, the NT scan plus bloodwork, runs weeks 11 to 13 and closes on schedule.
Most people call this the easy stretch, and relative to the neighbors, it is. Nausea recedes, hair thickens, and the bump arrives. The milestones are the good kind: hearing begins around week 16, first movements between weeks 18 and 22 for first-time parents (earlier if you've been here before), and the anatomy scan at 18 to 22 weeks, the longest and most detailed ultrasound of the pregnancy. At week 20, halfway, the baby averages about 10 inches crown to heel and 10.6 oz, roughly a banana. Week 24 brings the viability milestone and with it the glucose screening window (weeks 24–28). Symptoms shift rather than vanish: round ligament pain, nasal congestion, leg cramps, and the first Braxton Hicks tightenings.
Growth becomes the whole job: from about 2.2 lb at week 28 to 7.6 lb on average at week 40, near half a pound a week. Eyes open at 26, lungs mature through the final stretch, and brain development continues past the due date itself. The care rhythm steps up too: appointments every two weeks from 28, weekly from 36; daily kick counts from 28; the TDap vaccine between 27 and 36 weeks so antibodies reach the baby. ACOG divides the finish into early term (37–38 weeks), full term (39–40), late term (41), and postterm (42+), with 39 to 40 the ideal window. Only about 4 or 5 babies in 100 arrive on their exact due date; first babies skew a few days late.
| Week | Average length | Average weight | Comparison |
|---|---|---|---|
| 4 | 0.04 in | not yet | poppy seed |
| 6 | 0.13 in | not yet | lentil |
| 8 | 0.63 in | not yet | kidney bean |
| 10 | 1.2 in | 0.14 oz | prune |
| 12 | 2.1 in | 0.49 oz | lime |
| 14 | 3.4 in | 1.5 oz | lemon |
| 16 | 4.6 in | 3.5 oz | avocado |
| 18 | 5.6 in | 6.7 oz | bell pepper |
| 20 | ~10 in* | 10.6 oz | banana |
| 24 | 11.8 in | 1.32 lb | ear of corn |
| 28 | 14.8 in | 2.22 lb | eggplant |
| 32 | 16.7 in | 3.75 lb | jicama |
| 36 | 18.7 in | 5.78 lb | head of romaine |
| 40 | 20.2 in | 7.63 lb | small pumpkin |
*Lengths are crown-rump before week 20 and crown-heel from week 20, which is why the week 20 number appears to jump. These are commonly published averages from fetal growth charts; individual babies vary widely, and your ultrasound's measurements are always the ones that describe your baby.
Enter your LMP, due date, or conception date and get your gestational age, trimester, and this week's size, plus a page for every week 1–40.
Pregnancy Week-by-Week Tracker →The due-date question runs the whole timeline, and the due date calculator handles it from any direction, including IVF dates. Weight is the other number your provider tracks; the pregnancy weight gain calculator gives IOM ranges by week and pre-pregnancy BMI. And if you're still on the earlier side of the journey, the ovulation calculator maps the fertile window that started all this counting.
⚕️ Educational information only, not medical advice. Milestones and sizes are population averages. Symptoms that worry you deserve a call to your OB or midwife, not a chart.
Because the first day of the last menstrual period is a date most people can identify, while conception is invisible and typically happens about two weeks later. Gestational age therefore includes roughly two weeks when you weren't pregnant, which is why a full-term pregnancy is 40 gestational weeks but only about 38 weeks from conception.
It's a midpoint, not a deadline. Only about 4 to 5 in 100 babies arrive on their exact due date; most come within a week on either side, and first babies average a few days late. A first-trimester ultrasound re-dates a pregnancy more precisely than the LMP and can move the date by a week or more.
It depends on what hard means for you. Weeks 6 to 13 bring nausea and exhaustion; the second trimester is widely considered the easiest; the third brings heartburn, poor sleep, and physical weight. Medically, the first trimester carries the highest miscarriage risk and the third the highest preeclampsia and preterm-labor risk, so both get extra vigilance.
Early growth is explosive in percentage terms and tiny in grams. The baby goes from about 0.14 oz at week 10 to roughly 10.6 oz at the week 20 halfway point, then accelerates: about 1.3 lb at 24 weeks, 2.2 lb at 28, 4.2 lb at 33, and 7.6 lb on average at 40, with third-trimester gains near half a pound a week.