How this page is reviewed
| Risk tier | High YMYL |
|---|---|
| Author | Calculover Editorial Team Health education |
| Editorial owner | Calculover Reproductive Health Desk Pregnancy and fertility methodology owner |
| Reviewer | Calculover Editorial Review Medical-source review |
| Last reviewed | 2026-05-11 |
| Last verified | 2026-05-11 |
| Data effective date | 2026-05-11 |
Methodology
Pregnancy Trimester Resource follows the pregnancy or cycle formula described on the page, such as last-menstrual-period dating, IVF transfer dating, cycle-window estimation, hCG trend context, or pregnancy-weight categories. It keeps date and range outputs educational because clinical dating and pregnancy assessment require obstetric history and, often, ultrasound or lab follow-up.
Assumptions
- Cycle-based estimates assume the entered period dates, cycle length, luteal phase, and pregnancy dates are accurate and reflect the user rather than a population average.
- LMP due-date estimates generally assume a 28-day cycle with ovulation around day 14 unless the calculator provides alternate inputs such as ovulation, conception, ultrasound, or IVF transfer date.
- Pregnancy and fertility outputs assume a singleton pregnancy or typical cycle unless the user has entered data that the calculator specifically supports.
Limitations
- Irregular cycles, breastfeeding, postpartum changes, perimenopause, PCOS, fertility treatment, pregnancy loss, multiple gestation, and uncertain dates can make estimates inaccurate.
- Ovulation and fertile-window estimates are not reliable contraception and do not confirm pregnancy, miscarriage, ectopic pregnancy, fetal growth, or pregnancy viability.
- hCG levels and pregnancy-weight ranges vary widely; symptoms, bleeding, severe pain, high blood pressure, or concerning lab trends need prompt clinical review.
Sources
- Methods for Estimating the Due Date, American College of Obstetricians and Gynecologists
- Pregnancy Fact Sheet, Eunice Kennedy Shriver National Institute of Child Health and Human Development
- Fertility Awareness-Based Methods of Family Planning, American College of Obstetricians and Gynecologists
Professional guidance: Pregnancy Trimester Resource is for reproductive-health education and date planning only. It is not obstetric, fertility, contraceptive, diagnostic, or emergency medical advice; discuss results and symptoms with an obstetrician-gynecologist, midwife, fertility specialist, or other licensed clinician.
Pregnancy is divided into three trimesters: first (weeks 1-12), second (weeks 13-26), and third (weeks 27-40). Each involves distinct developmental milestones for the baby and physical changes for the mother.
The Three Trimesters
- First Trimester (Weeks 1-12): Embryo develops major organs, heartbeat begins (week 6), most critical period for avoiding harmful exposures. Common symptoms: nausea, fatigue, breast tenderness.
- Second Trimester (Weeks 13-26): Often called the "golden trimester." Baby grows rapidly, movements are felt (weeks 18-22), anatomy scan reveals sex. Morning sickness typically subsides.
- Third Trimester (Weeks 27-40): Baby gains weight and lung maturity. Braxton-Hicks contractions begin. Labor preparation starts. Baby is full-term at 39 weeks.
Real-World Example
A pregnancy with a last menstrual period of January 1: First trimester ends around March 26. Second trimester ends around June 25. Due date: approximately October 8 (40 weeks from LMP). Each trimester spans roughly 13 weeks.
Frequently Asked Questions
When does the second trimester start?
The second trimester begins at week 13 (the start of month 4). This is approximately 3 months after the first day of your last menstrual period.
Which trimester is most critical?
The first trimester is the most critical for development, as all major organs form during this time. The risk of miscarriage is highest in the first trimester. After 12 weeks, the risk drops significantly.
How is a due date calculated?
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. Only about 5% of babies are born on their exact due date; most arrive within 2 weeks before or after.