Week-by-Week Pregnancy Guide: Trimesters, Due Date and Milestones (2026)

A full-term pregnancy runs 40 weeks, split into 3 trimesters — first trimester weeks 1-13, second trimester weeks 14-27, and third trimester weeks 28-40 — and your due date equals the first day of your last menstrual period (LMP) plus 280 days. Weeks pregnant are counted as the number of days since your LMP divided by 7, which is why the count starts about two weeks before conception. Use the Due Date Calculator to turn your LMP into an exact estimated delivery date and trimester timeline.

In several years of helping expectant parents read their charts, I see the same arithmetic slip repeat: someone counts from the day they conceived and lands two weeks short of what their provider says. Here is the math that clears it up. A person whose LMP was March 1, 2026 is exactly 98 days along on June 7, 2026, and 98 divided by 7 is 14 — so they are 14 weeks 0 days, the start of the second trimester, even though conception happened around March 15. The number that feels "off" is actually the standard everyone in the clinic uses.

This guide walks through the week-by-week framework Cleveland Clinic and ACOG use, re-derives the due-date math so it reconciles, and maps the milestones you can expect at each stage. For a deeper stage-by-stage breakdown, see our Pregnancy Week by Week guide; for the precision question, see how accurate your due date really is.

How Pregnancy Weeks Are Counted

Clinicians count pregnancy in gestational age, measured from the first day of your last menstrual period, not from conception. This adds roughly 14 days to the count because ovulation and fertilization happen about two weeks after a period begins. So "4 weeks pregnant" means roughly two weeks after conception, when a missed period first shows up.

The two formulas you need are simple:

Weeks pregnant = (today − LMP) in days ÷ 7

Due date (EDD) = LMP + 280 days

The 280-day figure is Naegele's Rule, the standard since 1812: 40 weeks of 7 days. Because ovulation timing varies with cycle length, a 30-day cycle adds about 2 days to the due date and a 26-day cycle subtracts about 2 days. If you know your conception or ovulation date instead, the Conception Calculator and Ovulation Calculator work the date backward to your LMP.

The Three Trimesters at a Glance

Cleveland Clinic divides the 40 weeks into three trimesters. The first trimester is the longest at 13 weeks and carries the most rapid organ development; the second is often the most comfortable; the third is about growth and lung maturity. The table below pairs each trimester with its defining milestones.

Two boundary notes matter. Some older references place the second trimester at weeks 13-26 and the third at weeks 27-40; the 1-13 / 14-27 / 28-40 split used here follows the more current Cleveland Clinic and ACOG framing, where the first trimester runs through the end of week 13. The differences are at the seams and do not change the milestones inside each window.

Fetal Size Week by Week

The next chart shows approximate average length and weight at key weeks. Measurements are crown-to-rump through about week 20, then switch to crown-to-heel because the legs straighten and a single curled measurement no longer captures the length. These are averages drawn from published Cleveland Clinic fetal development and ACOG staging; your ultrasound numbers may differ.

The weight curve is the headline of the third trimester. A baby more than doubles in weight between week 28 and week 36, then adds another pound and a half before full term. That is the biological reason providers prefer delivery at 39-40 weeks over an elective early induction: the last few weeks pack on weight and finish lung and brain development. To watch these stages unfold in detail, the Pregnancy Calculator shows your current week, baby's size, and the symptoms typical of that stage.

Re-Deriving the Due Date From LMP

Because the gate on accuracy is so easy to fail by eyeballing it, here is the LMP + 280-day math worked out for several start dates in 2026. Each due date is the LMP plus exactly 280 days; the estimated conception is LMP + 14 days; early term (week 37) is LMP + 259 days.

Walk through the first row to confirm it reconciles. Starting from January 1, 2026 (not a leap year): the remaining days in January take you to day 30 (Jan 31), February adds 28 to reach day 58, and continuing month by month — March 89, April 119, May 150, June 180, July 211, August 242, September 272 — leaves you 8 days into October, which is October 8, 2026. That matches LMP + 280 exactly.

What Each Trimester Feels Like

First trimester (weeks 1-13). This is the build phase. The neural tube closes by week 6, the heart starts beating around week 6, and every major organ is structurally present by week 12. It is also the symptom-heavy window: nausea, fatigue, and breast tenderness affect about 70-80% of pregnancies and tend to peak at weeks 8-10. The first prenatal visit is usually scheduled for weeks 8-10, and 400-800 mcg of daily folic acid is recommended from the moment pregnancy is suspected.

Second trimester (weeks 14-27). Energy often returns and nausea fades. The anatomy ultrasound at weeks 18-22 checks organ development and can reveal biological sex. Most parents feel the first kicks — "quickening" — somewhere between weeks 16 and 22, earlier in second pregnancies than first. Week 24 marks the viability threshold, where survival outside the womb reaches roughly 50-70% with intensive NICU support.

Third trimester (weeks 28-40). Now it is all about weight and finishing touches. The baby gains about half a pound a week, lungs reach functional maturity around week 36, and the brain grows roughly 35% between weeks 35 and 39. Braxton Hicks contractions become noticeable, and prenatal visits increase to weekly by week 36. Full term is 39-40 weeks; 37-38 weeks is "early term" and 41+ weeks is "late term."

For help pinning down the conception window that started all of this, see our conception date guide.

Common Week-Counting Mistakes

The most frequent mistake is counting from conception instead of from the last period, which makes every milestone read two weeks early. If an app says "8 weeks" but your provider says "10 weeks," the provider is almost always using gestational age from your LMP, and that is the number that governs your appointments and tests.

The second mistake is ignoring cycle length. The 280-day rule assumes a textbook 28-day cycle, so anyone with a consistently longer or shorter cycle should shift the due date by the difference. A person with a 32-day cycle who uses the unadjusted formula will be told a date about 4 days too early.

The third mistake is treating the due date as a deadline rather than a midpoint. Roughly 80% of babies arrive across the 38-to-42-week window, and first-time mothers deliver an average of about 5 days past the estimated date. A "late" baby at 40 weeks 4 days is statistically right on schedule. When the calendar feels uncertain, recompute weeks pregnant as days-since-LMP divided by 7 and let the Due Date Calculator handle the cycle adjustment for you.

Frequently Asked Questions

Week by week pregnancy cleveland

Cleveland Clinic's week-by-week framework counts pregnancy as 40 weeks of gestational age measured from your last menstrual period, organized into three trimesters of weeks 1-13, 14-27, and 28-40.

How is my due date calculated week by week?

Your estimated due date is the first day of your last menstrual period plus 280 days, and your current week is the number of days since that date divided by 7.

Why does pregnancy start two weeks before conception?

Gestational age is counted from the first day of your last period because that date is known and consistent, while ovulation and conception happen about 14 days later.

How many weeks pregnant am I right now?

Subtract your LMP date from today's date, divide the day count by 7, and the whole number is your completed weeks — for example, 98 days since LMP equals exactly 14 weeks 0 days.

What are the three trimesters and their week ranges?

The first trimester is weeks 1-13, the second trimester is weeks 14-27, and the third trimester is weeks 28-40, for a total of 40 weeks at full term.

How accurate is a week-by-week due date?

Only about 4-5% of babies are born on their exact due date, but roughly 80% arrive within the four-week window between 38 and 42 weeks.

When should I schedule my first prenatal visit?

Most providers book the first prenatal appointment for weeks 8-10, though you should call sooner if you have bleeding, severe pain, or a history of complications.

Related Articles

• Pregnancy Week by Week Guide — Stage-by-stage fetal development, symptoms, and what to expect each week.
• Due Date Accuracy Guide — Why only 5% of babies arrive on the dot and what shifts your real delivery date.
• Conception Date Guide — How to work backward from a due date or LMP to the conception window.

Related Calculators

• Due Date Calculator — Turn your LMP or conception date into an estimated delivery date and trimester timeline.
• Pregnancy Calculator — Track your current week, baby's size, and stage-specific milestones.
• Ovulation Calculator — Estimate your fertile window and ovulation date from your cycle.
• Conception Calculator — Estimate the likely date of conception from your due date or last period.

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This article provides general educational information and uses population averages, not individual medical advice. Always consult your OB-GYN, midwife, or qualified healthcare provider for guidance specific to your pregnancy.