Pregnancy Trimester Calculator
Find your current trimester and its date ranges from your last menstrual period.
Enter the first day of your most recent period.
The date you want to check. Leave blank to use today.
Current Trimester
How Pregnancy Trimesters Work
Pregnancy is divided into three trimesters, each lasting roughly three months. The count begins on the first day of your last menstrual period (LMP), which is about two weeks before conception — the standard way clinicians measure gestational age. This calculator works out how many completed weeks pregnant you are on a given date and tells you which trimester that falls into, along with the calendar start date of each trimester.
The Three Trimesters
| Trimester | Weeks | Notes |
|---|---|---|
| First Trimester | Weeks 1 – 12 | Organs form; highest risk of early loss. |
| Second Trimester | Weeks 13 – 27 | Anatomy scan; movements usually felt. |
| Third Trimester | Weeks 28 – 40+ | Rapid growth; preparation for birth. |
Frequently Asked Questions
How is the trimester decided?
It is based on how many completed weeks pregnant you are. Weeks 0–12 are the first trimester, weeks 13–27 the second, and week 28 onward the third.
Why does counting start at the last period?
Gestational age is measured from the first day of the last menstrual period because that date is usually known, while the exact moment of conception is not.
Can I check a past or future date?
Yes. Use the "Calculate As Of" field to choose any date and see which trimester applies on that day. Leave it blank to use today.
Medical disclaimer: This calculator provides an estimate based on standard obstetric conventions and should not replace professional medical advice. A dating ultrasound performed by your healthcare provider is the most reliable way to confirm your gestational age and trimester. Individual pregnancies vary. Always consult your doctor or midwife for personalised guidance.