Common trimester ranges
| Stage | Common week range |
|---|---|
| First trimester | Week 1 through the end of week 13 |
| Second trimester | Week 14 through the end of week 27 |
| Third trimester | Week 28 through delivery |
Some medical sources describe the boundaries with slightly different day-level precision. That does not mean the pregnancy date itself changed.
Why week one begins before conception
Gestational age is traditionally counted from LMP, about two weeks before conception in a typical cycle. The first two numbered weeks therefore describe the dating convention rather than embryonic development after fertilization.
First trimester
Early pregnancy includes implantation and rapid organ development. Prenatal care establishes history, dating, testing plans, and individual guidance. Symptoms vary widely and cannot be predicted by a trimester label.
Second trimester
This middle stage includes continued growth and commonly scheduled prenatal assessments. Some people notice changes in energy or movement, while experiences remain individual.
Third trimester
The final stage includes substantial growth and preparation for delivery. Prenatal visit timing and recommendations depend on individual health and pregnancy history.
How the calculator assigns a trimester
The due-date calculator estimates gestational age from the selected dating method and labels weeks below 14 as first trimester, weeks 14–27 as second, and week 28 onward as third.
A trimester is not a medical assessment
The label cannot determine whether symptoms are normal, evaluate fetal development, or replace prenatal care. Contact a qualified clinician for pregnancy-specific concerns.
Frequently asked questions
When does the second trimester start?
WellnessCalcs uses week 14. Some sources express boundaries with slightly different day-level wording.
When does the third trimester start?
WellnessCalcs uses week 28.
Why do apps show different trimesters?
They may use different boundary conventions or a different estimated due date.
Is pregnancy exactly 40 weeks?
Forty weeks is the standard estimated timeline from LMP, not a guarantee of delivery on one day.
Sources
References and further reading
- American College of Obstetricians and Gynecologists — How Your Fetus Grows During Pregnancy
- Office on Women’s Health — Stages of Pregnancy
Last reviewed: July 13, 2026