Estimate Weeks Pregnant & Due Date
Enter the first day of your last menstrual period (LMP) and your average cycle length to estimate your gestational age.
How Pregnancy Dating Works
Pregnancy is clinically defined as lasting approximately 40 weeks (280 days) from the first day of your last menstrual period (LMP). In clinical settings, obstetricians date pregnancy this way because the day of your period is a clear physical marker, whereas the exact day of ovulation or conception is harder to determine. Because ovulation typically occurs around 14 days into your cycle (for a standard 28-day cycle), you are technically not pregnant during the first two weeks of gestation.
LMP vs. Ultrasound Fetal Dating
Dating a pregnancy using the LMP relies on the assumption of a regular 28-day cycle with ovulation occurring exactly at the midpoint. However, cycle lengths fluctuate naturally, and many women ovulate earlier or later than day 14. If you have an irregular cycle, calculation from the last period may suggest the baby is older or younger than it is.
To establish the most accurate dating timeline, healthcare providers perform a first-trimester ultrasound (usually between 8 and 14 weeks). The sonographer measures the crown-rump length (CRL) of the embryo. First-trimester ultrasound dating is highly reliable and has a margin of error of only 3 to 5 days. If there is a discrepancy of more than 7 days between LMP dating and the first-trimester ultrasound, the due date is adjusted to match the ultrasound measurements.
What Can Affect Due Date Estimates
- Cycle Irregularity: Cycles shorter than 21 days or longer than 45 days shift ovulation timings dramatically, making LMP estimates less reliable.
- Recent Hormonal Contraception Use: If you stopped taking oral contraceptives recently, your first post-pill cycles may be longer or irregular, making dating from LMP inaccurate.
- Recalling LMP Dates: Incorrectly recalling the exact start day of your last period is highly common and shifts estimates.
- Multiple Gestation: Carring twins or triplets does not shift the gestational age calculation, but it often alters the clinical delivery timeframe.
What to Track Before Your Prenatal Appointment
To prepare for your first clinical booking appointment, note these details to share with your obstetrician, midwife, or nurse practitioner:
- The exact start date of your LMP: Specifically, the first day of active flow, not light spotting.
- Your cycle history: Whether your cycles are generally regular, their average length, and if you recently used birth control.
- Positive test dates: Note the dates of any positive home pregnancy tests.
- Medical milestones: Note the dates of any spotting, cramping, or severe morning sickness.
Frequently Asked Questions
Gestational age is calculated from the first day of your last menstrual period (LMP). Even though you are not pregnant during the first two weeks of this timeframe (before ovulation occurs), clinical practice standards date pregnancy from the LMP to maintain a consistent standard.
Pregnancy calculators provide a statistical estimate based on an average 28-day cycle. Individual cycle variations, exact ovulation timing, and physical factors mean this date is only an estimate. Your obstetrician or midwife will verify dating via a first-trimester ultrasound.
No. This tool calculates dates assuming you are already pregnant. To confirm pregnancy, take a home urine pregnancy test or consult a healthcare professional for a blood test and clinical evaluation.
If your cycles are irregular or vary significantly, LMP dating is less reliable because the exact day of ovulation is harder to predict. In such cases, a first-trimester dating ultrasound is considered the most accurate method for establishing gestational age.
It is recommended to schedule your first prenatal appointment (often called a booking visit) around 8 weeks after your LMP. However, contact your doctor as soon as you get a positive pregnancy test to discuss prenatal vitamins (like folic acid) and safe habits.
Seek emergency medical attention immediately if you experience severe abdominal or pelvic pain, heavy vaginal bleeding, persistent vomiting that prevents hydration, sudden severe swelling in your hands or face, extreme headaches, vision changes, or high fever.
