Interactive Obstetric Calculator
Enter the first day of the last menstrual period (LMP) to estimate due date, current gestational age, and key timeline milestones.
Educational tool only. This obstetric calculator does not replace professional medical assessment or ultrasound dating.
What this obstetric calculator does
An obstetric calculator helps estimate where a pregnancy sits on the timeline. In day-to-day use, clinicians and patients usually want fast answers to questions like: “How many weeks am I?”, “When is my due date?”, and “What milestone comes next?” This tool is designed for those practical questions.
It uses the common clinical starting point: the first day of the last menstrual period (LMP). From there, it calculates estimated date of delivery (EDD), gestational age on a chosen date, and trimester status.
Core outputs you get
- Estimated due date (EDD): projected date around 40 weeks gestation.
- Estimated conception date: approximated from ovulation timing.
- Current gestational age: weeks and days from LMP to your selected date.
- Trimester: first, second, or third trimester classification.
- Timeline markers: end of first trimester, full-term window, and post-term threshold.
How the calculations work
The calculator uses a standard pregnancy dating model (similar to Naegele-based dating):
- Base pregnancy duration: 280 days from LMP.
- Cycle-length adjustment: if your cycle differs from 28 days, due date shifts by that difference.
- Gestational age: number of days between LMP and the selected “as of” date, expressed in weeks + days.
Why cycle length matters
If ovulation usually occurs later (for example, with a 32-day cycle), conception may happen a few days later than a 28-day cycle assumption. The calculator reflects that by adjusting the EDD. This can improve rough estimates before definitive ultrasound dating is available.
How to use the tool accurately
- Enter the first day of your last period, not the day bleeding ended.
- Enter your average cycle length honestly (use 28 if unsure).
- Choose an “as of” date (today is usually best for current status).
- Click Calculate and review timeline outputs.
If your cycles are irregular, if conception date is known (for example, IVF transfer), or if early ultrasound differs significantly from LMP dating, clinical dating rules may override this estimate.
Pregnancy timeline quick guide
First trimester (0w0d to 13w6d)
Organ development is rapid. Many people complete initial prenatal labs and first-trimester screening during this window.
Second trimester (14w0d to 27w6d)
Often called the most physically comfortable phase. Anatomy ultrasound is commonly performed around 18–22 weeks.
Third trimester (28w0d onward)
Focus shifts to fetal growth, maternal monitoring, delivery planning, and signs of labor. Term pregnancy usually begins around 39 weeks.
Important limitations
- This obstetric calculator provides estimates, not diagnosis.
- Ultrasound dating (especially early pregnancy) can refine or replace LMP estimates.
- Symptoms such as pain, bleeding, severe headache, reduced fetal movement, or fluid leakage require prompt medical evaluation.
Frequently asked questions
Is the due date exact?
No. It is an estimate. Birth commonly occurs before or after the calculated date.
What if I do not remember my LMP?
Use the best estimate you have, then confirm with prenatal care and ultrasound. Clinical teams can provide more accurate dating methods when LMP is uncertain.
Can this be used for medical decisions?
Use it for planning and education only. Clinical decisions should always be made with your obstetric provider.