If you want a quick estimate of how far along a pregnancy is, this pregnancy week calculator by last menstrual period (LMP) can help. Enter the first day of your last period, your usual cycle length, and an “as of” date to instantly see gestational age, current pregnancy week, and estimated due date.
Pregnancy Week Calculator (LMP Method)
How this LMP pregnancy calculator works
The LMP method dates pregnancy from the first day of your last menstrual period, not from conception. This is standard in obstetrics because it creates a consistent reference point, even though conception usually occurs about two weeks later.
Formula used by this calculator
- Gestational age (days) = As-of date − LMP date
- Gestational age (weeks + days) = total days converted to weeks and remaining days
- Estimated due date (EDD) = LMP + 280 days + cycle adjustment
- Cycle adjustment = (your cycle length − 28 days)
- Estimated conception date ≈ LMP + (cycle length − 14)
How to use this tool correctly
Step-by-step
- Enter the exact first day of your most recent period.
- Enter your typical cycle length if you track it (leave 28 if uncertain).
- Choose the date you want to calculate for (today is auto-filled).
- Click Calculate Pregnancy Week.
You’ll get your current gestational age, the pregnancy week you are in, trimester, estimated conception date, and due date countdown.
Understanding pregnancy weeks and trimesters
Pregnancy is commonly described as 40 weeks (280 days), divided into three trimesters:
- First trimester: week 1 through 13 weeks + 6 days
- Second trimester: week 14 through 27 weeks + 6 days
- Third trimester: week 28 to birth
When your result says, for example, 18 weeks 3 days, that means you have completed 18 full weeks and are currently in pregnancy week 19.
How accurate is an LMP due date?
The LMP method is useful and widely used, but it can be less accurate when cycles are irregular, ovulation timing varies, or LMP date is uncertain. In many pregnancies, an early ultrasound provides more precise dating and may adjust the due date.
Common reasons dates change
- Irregular menstrual cycles
- Recent hormonal contraception changes
- Uncertain recall of period start date
- Late or early ovulation
When to contact your care team
Contact your obstetrician, midwife, or clinic if you have bleeding, severe pain, dizziness, persistent vomiting, fever, or any symptoms that worry you. If your dates from this calculator differ from your ultrasound dating, follow your clinician’s guidance.
Frequently asked questions
What if I do not remember my exact LMP date?
Use your best estimate and schedule clinical dating. Ultrasound is typically the better method when LMP is uncertain.
Does this calculator tell me the exact day I conceived?
No. It gives an estimate based on cycle assumptions. Actual conception can vary by several days.
Can cycle length really affect due date?
Yes, if your cycles are consistently longer or shorter than 28 days, ovulation timing may shift. This tool applies that adjustment for a better estimate.
Bottom line
This pregnancy week calculator by last menstrual period is a practical way to estimate gestational age and due date at home. Use it for planning and understanding your timeline, then confirm dating and prenatal milestones with your healthcare professional.