Trimester Schedule & Boundaries
Trimester Calculator
Find out which trimester you are in today and view the projected calendar dates for each stage of your pregnancy.
What Trimester Am I In?
Enter your LMP date to identify your current trimester and view your personalized 3-trimester date timeline.
Your Current Stage
Second Trimester
Weeks 14 through 27. Often called the golden trimester as energy returns.
Your Trimester Timeline
Completed
First Trimester
Weeks 1â13 (0w 0d to 13w 6d)
From: June 10, 2026
Until: September 15, 2026
From fertilization through organogenesis and the 12-week development milestone.
Active Trimester
Second Trimester
Weeks 14â27 (14w 0d to 27w 6d)
From: September 16, 2026
Until: December 22, 2026
Rapid fetal growth, emergence of coordinated movements, and the 20-week anatomy scan.
Upcoming
Third Trimester
Weeks 28â40+ (28w 0d to delivery)
From: December 23, 2026
Until: March 17, 2027
Subcutaneous fat gain, lung maturation, and preparation for labor and delivery.
A note on trimester dividing lines: In medical practice, exact trimester boundaries can vary slightly by clinical institution (some sources divide trimesters evenly at 13 weeks 2 days, while others, like ACOG, place the transition at completed Week 13 and completed Week 27). This tool uses the standard ACOG/NHS convention. Trimesters are clinical landmarks rather than rigid biological boundaries.
Understanding the Trimester Model
Pregnancy is traditionally divided into three developmental segments called trimesters. Because human pregnancy lasts approximately 40 weeks (280 days), which does not divide evenly into three whole numbers, obstetrical organizations use standard week grouping conventions:
First Trimester: Weeks 1 through 13 (Days 0â97)Begins on the first day of your LMP. Includes conception, blastocyst implantation, early placental development, and the formation of all major organs.
Second Trimester: Weeks 14 through 27 (Days 98â195)Often accompanied by renewed energy as early nausea subsides. Baby grows rapidly, reflexes strengthen, and the mid-pregnancy anatomy ultrasound is typically scheduled around Week 20.
Third Trimester: Weeks 28 through 40+ (Days 196â280+)Focuses on final fetal maturation, significant weight gain, lung development with surfactant production, and preparation for birth.
Common Questions
Pregnancy Calculation FAQ
Reliable, evidence-based answers to frequently asked questions about gestational dating and timelines.
How is a pregnancy due date calculated?
Medical providers calculate estimated due dates using Naegele's rule: by adding 280 days (40 weeks) to the first day of your last normal menstrual period (LMP). If your menstrual cycle is regularly longer or shorter than the standard 28-day model, the calculation can be adjusted accordingly.
How many weeks is a full-term pregnancy?
A full-term pregnancy is generally considered 40 weeks (280 days) from your LMP. Medical guidelines classify babies born between 37 weeks and 38 weeks and 6 days as 'early term', between 39 weeks and 40 weeks and 6 days as 'full term', and 41 weeks to 41 weeks and 6 days as 'late term'.
How are pregnancy weeks counted?
Pregnancy weeks describe completed weeks plus extra days. For example, if you are at '12 weeks, 3 days', you have completed 12 full weeks and are currently experiencing your 13th week of pregnancy.
What is the difference between gestational age and conception age?
Gestational age is measured from the first day of your last period, before ovulation and fertilization actually occur. Conception age (fetal age) begins at the moment the egg is fertilized, which typically takes place approximately 14 days after your period. Gestational age is standard in medical obstetrics.
Can my healthcare provider give me a different due date?
Yes, and this is completely normal. An early first-trimester ultrasound dating scan (crown-rump length measurement) provides the most precise estimate of fetal development. If there is a notable discrepancy between your LMP date and the ultrasound measurement, your clinician will update your official estimated due date.
How accurate is an estimated due date?
Only about 4% to 5% of babies arrive precisely on their estimated due date. It is best considered an estimated arrival window; roughly 80% of healthy infants are born within a two-week window before or after the estimated date.