Pregnancy Calculator — Due Date and Pregnancy Week Tracker
Calculate your pregnancy due date, current week, and trimester. Free pregnancy due date calculator based on your last menstrual period.
A pregnancy calculator estimates your baby's due date and tracks how far along you are based on your last menstrual period (LMP). While only about 4% of babies arrive on the exact predicted due date, the estimate gives you a meaningful timeline for prenatal care, work planning, and personal preparation.
Our calculator uses the standard Naegele's rule, the same method used by doctors and midwives worldwide. Enter the first day of your last menstrual period, and you will get your estimated due date, current pregnancy week, current trimester, and how many days remain.
How due date is calculated
Naegele's rule calculates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day menstrual cycle with ovulation on day 14, and counts from before conception actually occurred.
Pregnancy is divided into three trimesters: first trimester runs weeks 1-12, second trimester weeks 13-26, and third trimester weeks 27-40. Each trimester brings distinct physical changes for the mother and developmental milestones for the baby.
Babies are considered full-term between 37 and 42 weeks. Births before 37 weeks are preterm, and births after 42 weeks are post-term — both monitored carefully by healthcare providers.
What happens in each trimester
First trimester (weeks 1-12): The baby develops from a single cell into a small fetus with all major organs forming. Mothers often experience morning sickness, fatigue, and breast tenderness. The first prenatal visit usually happens around week 8-10.
Second trimester (weeks 13-26): Often the most comfortable stage. Morning sickness typically eases, energy returns, and the baby bump becomes visible. The baby's movements become noticeable around weeks 18-22. The 20-week ultrasound checks anatomy and often reveals the baby's sex.
Third trimester (weeks 27-40): The baby grows rapidly and prepares for birth. Mothers may experience back pain, swelling, and difficulty sleeping. Prenatal visits become more frequent (every 2 weeks, then weekly near the end). Hospital tour and birth plan finalization typically happen in this stage.
How to use this calculator
- Find the first day of your last menstrual period.
- Enter that date in the calculator.
- Click Calculate to see your due date, current pregnancy week, and trimester.
Worked example: LMP January 1
Adding 280 days to January 1 gives an estimated due date of October 8.
Current week is calculated from the LMP date, so on March 15 (10 weeks after LMP), you would be 10 weeks pregnant — early second trimester is just around the corner.
Frequently asked questions
How accurate is the due date?
Only about 4% of babies are born on the exact due date. About 80% arrive within 2 weeks before or after. The due date is a useful target, not a guarantee.
What if my cycle is not 28 days?
Standard calculators assume 28 days. If your cycle is significantly longer or shorter, your actual conception date — and due date — may shift. An early ultrasound (8-12 weeks) gives a more accurate estimate.
Can I trust the LMP method if I conceived shortly after stopping birth control?
Cycles can be irregular for several months after stopping hormonal birth control, making LMP-based dating less reliable. An early ultrasound is more accurate in this case.
When should I call my doctor?
Schedule your first prenatal visit as soon as you confirm pregnancy, typically 8-10 weeks. Call sooner for severe symptoms like heavy bleeding, severe pain, or persistent vomiting.