Day 0
Current Week
Week 33weeks
Scenario
Pregnancy Weeks by Trimester — Complete Week-by-Week Guide
Understand all 40 weeks of pregnancy divided into 3 trimesters. First trimester (1-12), second (13-27), third (28-40) with key milestones at each stage.
Interactive Calculator
Your Pregnancy Details
First day of your last period. Gestational age is calculated from this date.
Your Pregnancy Timeline
Trimester
Weeks 1–13
~38 weeks
Days Remaining
266days
Week 15% completeWeek 40
0
4
8
12
16
20
24
28
32
36
40
Weeks 1–13
Major organs and systems form. Common symptoms include fatigue, nausea, and breast tenderness.
Tri 2
Second Trimester
Weeks 14–27
Baby grows rapidly. Movement felt. Symptoms often improve. Anatomy scan at 18–22 weeks.
Tri 3
Third Trimester
Weeks 28–40+
Baby continues maturing. Braxton Hicks, back pain, and nesting instinct are common.
Pregnancy Milestones
✓
Conception
Week 0
✓
Embryo Implants
Week 2
6
Heartbeat Detectable
Week 6
12
NT Scan
Week 12
20
Anomaly Scan
Week 20
24
Viability Threshold
Week 24
28
Third Trimester Begins
Week 28
37
Full Term
Week 37
40
Due Date
Week 40
You are in the First Trimester (Weeks 1–13). You are week 3, day 0 of your pregnancy. Approximately 266 days (38 weeks) remain until your due date.
The formula
The formula
What Gestational Age Measures
Pregnancy is measured in gestational age, which counts from the first day of your last menstrual period (LMP), not from the date of conception. This is the standard method used by obstetricians and midwives worldwide because the LMP date is typically the most reliably remembered reference point. A full-term pregnancy is defined as 40 weeks (280 days) from LMP, though only about 5% of babies are born on their exact due date.
Gestational age is the universal reference for scheduling prenatal tests, assessing fetal development milestones, determining viability, and planning delivery. Understanding your current pregnancy week helps you know what to expect at each stage of your pregnancy journey and ensures timely prenatal care.
The Gestational Age Formulas
Days Since LMP = Today − LMP
Current Week = Floor(Days Since LMP / 7) + 1
Current Day = Days Since LMP mod 7
Days Until Due Date = 280 − Days Since LMP
LMP = first day of last menstrual periodFloor = round down to nearest integer
Formula Source
This calculator uses the **standard gestational age calculation (days from LMP to today converted to weeks)** from **ACOG (American College of Obstetricians and Gynecologists)** guidelines.
Reference URL: https://www.acog.org/womens-health/infographics/pregnancy-timeline
Last Verified: 2026-07-30
Worked Example
If your LMP was January 1, 2026 and today is August 9, 2026: Days since LMP = 220 days. Current Week = Floor(220 / 7) + 1 = Floor(31.4) + 1 = 31 + 1 = 32. Current Day = 220 mod 7 = 3. This means you are 32 weeks and 3 days pregnant. Days until due date = 280 − 220 = 60 days remaining (approximately 8 weeks and 4 days). Your due date calculated from Naegele's Rule (LMP + 280 days) would be October 8, 2026.
Trimester Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | Weeks 1–13 | Organ formation, heartbeat detectable, neural tube closure |
| Second Trimester | Weeks 14–27 | Growth, fetal movement felt, anatomy scan, viability threshold |
| Third Trimester | Weeks 28–40+ | Rapid weight gain, lung maturation, head-down positioning, labor preparation |
FAQ-Style Explanations
What is the difference between gestational age and fetal age? Gestational age is measured from LMP (about 40 weeks total) and is the standard medical reference. Fetal age (or conceptual age) is measured from conception and is about 2 weeks less than gestational age. This calculator uses gestational age, which is what your healthcare provider will use.
What if I have irregular cycles? For people with regular 28-day cycles, LMP-based calculation is reasonably accurate. However, if you have irregular cycles or are unsure of your LMP, first-trimester ultrasound dating is more reliable. Crown-rump length (CRL) measurement between 7–13 weeks is considered the most accurate method for dating a pregnancy, with an error margin of only ±3–5 days.
Is it normal for my calculated weeks to differ from my ultrasound? Yes, it is common for LMP-based calculations to differ from ultrasound dating. If the difference is more than 5–7 days in the first trimester, most providers will use the ultrasound date as the reference. Discrepancies often arise from irregular ovulation timing or inaccurate LMP recall.
Known Limitations
- Assumes a regular 28-day menstrual cycle with ovulation on day 14. Cycles shorter or longer than 28 days will shift the actual ovulation date and therefore the true gestational age.
- Relies on accurate recall of the LMP date, which may not be reliable for all individuals.
- Does not account for variations in the luteal phase length, which can range from 10–16 days across different individuals.
- For IVF pregnancies, the gestational age should be calculated from the transfer date and embryo age, not from LMP.
- The 40-week (280-day) standard is an average — normal pregnancies range from 37 to 42 weeks. Post-term pregnancies beyond 42 weeks may require medical intervention.
- First-trimester ultrasound dating is considered more accurate than LMP-based calculation and is preferred when available, especially for clinical decision-making.
Scenario guide
Scenario guide
First Trimester — Weeks 1 to 12
The first trimester spans weeks 1 through 12 and accounts for nearly all of the major organ formation that defines a healthy pregnancy. By week 4, the fertilized egg has implanted into the uterine lining, and human chorionic gonadotropin (hCG) levels begin the rapid rise that triggers a positive home pregnancy test. The heart begins beating around week 6, detectable by transvaginal ultrasound at about 70 beats per minute. The American College of Obstetricians and Gynecologists (ACOG) notes that the first trimester carries the highest miscarriage risk, estimated at roughly 10% of all recognized pregnancies, with the vast majority occurring before week 12. Most individuals experience morning sickness, fatigue, and breast tenderness during this period, and the fetal heart is visible on ultrasound by week 6.
Second Trimester — Weeks 13 to 27
The second trimester runs from week 13 through week 27 and is widely considered the most comfortable phase of pregnancy. Morning sickness typically resolves by week 16, and energy levels rebound significantly. The milestone most people look forward to is quickening — the first perception of fetal movement — which usually occurs between weeks 16 and 20, depending on whether this is a first pregnancy or a subsequent one. The detailed anatomy ultrasound, scheduled between weeks 18 and 22, screens for structural anomalies and confirms fetal growth parameters. By week 24, a fetus is considered potentially viable with intensive neonatal intensive care unit (NICU) support, and survival rates exceed 60% at this gestational age. The glucose screening test, recommended between weeks 24 and 28, checks for gestational diabetes.
Third Trimester — Weeks 28 to 40
The third trimester begins at week 28 and ends at week 40, covering the final stretch before delivery. During weeks 28 to 32, the baby gains approximately 200 to 250 grams per week and the lungs produce surfactant, a protein that prevents alveolar collapse and is critical for breathing after birth. By weeks 33 to 36, the baby typically engages — meaning the head drops into the pelvis — and Braxton Hicks contractions become more frequent and noticeable. From week 37 onward, the pregnancy is considered early-term, and from week 39 to 40 it is full-term, which is the optimal window for delivery. After week 40, the pregnancy is late-term, and after week 42 it is post-term, at which point most obstetricians recommend induction.
Key Developmental Milestones by Month
Tracking fetal development month by month helps expectant parents understand what to expect at each stage. The fetal heartbeat is typically visible by week 6 and audible by Doppler around week 12. Fetal movement is first observed on ultrasound around week 10 but is not felt by the mother until weeks 16 to 20. By week 18, the baby can hear sounds from outside the womb, and the World Health Organization (WHO) recommends regular prenatal checkups to monitor this development. Lung maturity, as confirmed by amniotic fluid lecithin-sphingomyelin ratios, is usually reached by week 34. By week 36, the baby has gained most of its birth weight and has completed the descent into the birth canal. These milestones guide prenatal care schedules and help identify pregnancies that may need additional monitoring.
Full-Term and Post-Term Definitions
The American College of Obstetricians and Gynecologists (ACOG) defines full-term pregnancy as delivery between 39 weeks 0 days and 40 weeks 6 days, which carries the lowest risk of complications for the newborn. Early-term covers 37 weeks 0 days through 38 weeks 6 days and is associated with slightly higher rates of respiratory difficulty. Late-term spans 41 weeks 0 days through 41 weeks 6 days, and post-term begins at 42 weeks 0 days. After 42 weeks, the placenta's efficiency declines, amniotic fluid volume may drop, and the risk of meconium aspiration and macrosomia increases. Most obstetric guidelines recommend active monitoring and discussion of induction between weeks 41 and 42 to avoid post-term complications.
FAQ
Frequently Asked Questions
Pregnancy is divided into three trimesters: the first trimester covers weeks 1 to 12, the second trimester covers weeks 13 to 27, and the third trimester covers weeks 28 to 40. The first trimester is when all major organs form and carries the highest miscarriage risk at approximately 10%. The second trimester is generally the most comfortable, with morning sickness resolving and quickening (first fetal movement) occurring between weeks 16 and 20. The third trimester is when the baby gains most of its birth weight and prepares for delivery, with lung maturation typically complete by week 34.
Key first-trimester milestones include implantation around week 4, detectable hCG levels from about week 4 to 5, visible fetal heartbeat on ultrasound around week 6, and the beginning of fetal features forming by week 8. By week 12, all major organs have formed and the risk of miscarriage drops below 3%. The nuchal translucency (NT) scan, which screens for chromosomal abnormalities, is performed between weeks 11 and 14. Morning sickness typically peaks around week 9 and begins resolving by week 14 to 16.
Quickening — the first perception of fetal movement — typically occurs between weeks 16 and 20. First-time mothers usually feel it later, around weeks 18 to 20, because they have less experience distinguishing fetal movement from gas or other sensations. Mothers who have been pregnant before often feel quickening earlier, sometimes as early as week 16. By the third trimester, fetal movement becomes regular and distinct, and counting kick counts becomes an important self-monitoring practice.
ACOG defines full-term pregnancy as delivery between 39 weeks 0 days and 40 weeks 6 days. Early-term covers 37 weeks 0 days to 38 weeks 6 days, late-term covers 41 weeks 0 days to 41 weeks 6 days, and post-term begins at 42 weeks. The full-term window (39 to 40 weeks) is associated with the lowest risk of neonatal complications. After 42 weeks, placental efficiency declines and risks of meconium aspiration and macrosomia rise, which is why most providers recommend induction between weeks 41 and 42.
The third trimester, particularly weeks 28 to 34, is critical for lung development because this is when surfactant production ramps up. Surfactant is a protein-lipid substance that lines the alveoli and prevents them from collapsing after each breath. Without adequate surfactant, a newborn can develop respiratory distress syndrome (RDS). Lung maturity, as assessed by amniotic fluid lecithin-sphingomyelin ratios, is typically achieved by week 34. For this reason, avoiding delivery before 39 weeks unless medically necessary is strongly recommended by ACOG.
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