Pregnancy trimester calculator
Enter the first day of your last menstrual period (LMP) to see your estimated due date, your gestational age in weeks and days, and the exact dates each trimester begins and ends. It is a quick way to know how far along you are and what stage of pregnancy you are in today.
How it works
Dating follows the standard Naegele’s rule and fixed trimester boundaries, counted in gestational weeks from the LMP:
due date = LMP + 280 days (40 weeks)
gestational age = days since LMP, shown as weeks + days
first trimester = weeks 0–13 (up to day 97)
second trimester = weeks 14–27 (days 98–195)
third trimester = week 28 onward (day 196 to birth)Example
For an LMP, with 98 days elapsed (14 weeks 0 days):
- Gestational age: 14 weeks 0 days
- Stage: Second trimester (just started)
- Due date: LMP + 280 days
| Gestational age | Trimester |
|---|---|
| 0–13 weeks | First |
| 14–27 weeks | Second |
| 28 weeks to birth | Third |
What happens in each trimester
First trimester (weeks 1–13)
The most critical period for fetal development. All major organs begin forming during the embryonic phase (weeks 1–8), and by week 10 the embryo becomes a fetus with a beating heart, recognisable limb buds, and developing facial features. Nausea, fatigue, and breast tenderness are common in this period. Most miscarriages occur during the first trimester.
Key appointments typically include a first prenatal visit, blood typing and screening tests, and the first ultrasound (often at 8–12 weeks to confirm dating and check the heartbeat).
Second trimester (weeks 14–27)
Often called the “golden trimester” because early symptoms usually ease and energy returns. The fetus grows rapidly — from about 9 cm at week 14 to roughly 35 cm by week 27. Fetal movements (quickening) are typically felt between weeks 16 and 22 in a first pregnancy, sometimes earlier in subsequent ones. Anatomy scans (usually around 20 weeks) check for structural developments and can reveal the baby’s sex.
Third trimester (weeks 28 to birth)
The fetus gains most of its birth weight and the lungs mature. Third-trimester complications such as preeclampsia, gestational diabetes, and preterm labour are monitored through regular antenatal appointments. After week 37 the pregnancy is considered term; after week 42 it is post-term and clinical management is typically offered.
LMP dating versus ultrasound dating
Naegele’s rule is an estimate based on a 28-day cycle and ovulation on day 14. In practice, cycle lengths vary, and conception may not have occurred exactly two weeks after the LMP. Early ultrasound (ideally before 14 weeks) provides a more precise gestational age based on fetal measurements and is considered more accurate when the two differ. If your clinician adjusts your due date based on ultrasound, use that revised date — the ultrasound-derived due date is the one that matters clinically.
Ultrasound dating may shift these dates, so use this tool as a guide and confirm details with your healthcare provider. Everything is computed privately in your browser.