How to Use the Pregnancy Due Date Calculator
The Pregnancy Due Date Calculator estimates your baby's estimated due date (EDD) from three different starting points: your last menstrual period (LMP), the date of conception, or an ultrasound measurement. It also shows your current trimester and key milestone dates throughout the pregnancy.
Select your calculation method and enter the relevant date. From LMP: the EDD is typically 280 days (40 weeks) from the first day of your last period. From conception: add 266 days (38 weeks). From ultrasound: uses the measured gestational age. All three methods are standard obstetric practice.
An important nuance: the EDD is an estimate — only about 5% of babies are born exactly on their due date. The normal range is 37–42 weeks gestation. Most healthcare providers induce labour around 41–42 weeks if birth hasn't occurred naturally. Early ultrasound dates (before 13 weeks) are the most accurate for dating a pregnancy.
📊 Worked Example
LMP: 15 January 2025:
- Estimated Due Date: 22 October 2025
- End of 1st trimester (12 weeks): 9 April 2025
- End of 2nd trimester (27 weeks): 30 July 2025
- Viability milestone (24 weeks): 2 July 2025
- Term begins (37 weeks): 24 September 2025
Common Use Cases
- ✅ Calculating your estimated due date from your last period
- ✅ Confirming due date from conception date (IVF or known ovulation)
- ✅ Working out pregnancy milestones and trimester dates
- ✅ Planning maternity leave start date around the due date
- ✅ Tracking how many weeks pregnant you are today
- ✅ Understanding the 'full term' window (37–42 weeks)
Frequently Asked Questions
How accurate is a due date calculated from LMP?
Due dates from LMP assume a regular 28-day cycle with ovulation on day 14. For women with irregular or longer/shorter cycles, the LMP date may be less accurate. An early ultrasound (before 13 weeks) measures crown-rump length (CRL) and can date the pregnancy within ±5 days — significantly more accurate than LMP for irregular cycles.
What are the three trimesters of pregnancy?
First trimester: weeks 1–12 (conception through 3 months). Second trimester: weeks 13–27 (the 'honeymoon' period, energy returns, bump visible). Third trimester: weeks 28–40+ (rapid baby growth, preparing for birth). Each trimester has distinct developmental milestones and screening tests.
What does 'gestational age' mean?
Gestational age is counted from the first day of your last menstrual period — not from conception. At the time of conception (typically week 2 of a 28-day cycle), you're considered 2 weeks gestational age. This is why a 'full-term' pregnancy is 40 weeks gestational age, even though the embryo/fetus is only 38 weeks old from conception.
Can my due date change?
Your midwife or doctor may adjust your due date based on dating ultrasound findings. If your first ultrasound (dating scan, typically at 10–13 weeks) shows a different gestational age than your LMP suggests, the ultrasound date is usually preferred. Subsequent ultrasounds (anomaly scan at 20 weeks) are not used to change the due date.
What is the significance of 37 weeks?
37 weeks is the threshold for 'full term' — the baby is considered ready for birth with a very low risk of serious complications from prematurity. Babies born at 37–38 weeks are 'early term' and may need slightly more support than those born at 39–40 weeks. Most obstetricians prefer delivery between 39–41 weeks when possible.