Pregnancy Calculator
Calculate your gestational age, current trimester, and birth countdown.
Key Milestones
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. Gestational age from a last menstrual period assumes a regular 28-day cycle. Antenatal care should follow the dating from your own ultrasound and midwife or obstetrician, not this estimate.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently Asked Questions
How is gestational age calculated in pregnancy?
Gestational age is measured from the first day of your last menstrual period (LMP), even though conception occurs roughly two weeks later.
What are the three pregnancy trimesters?
First Trimester: Weeks 1 to 13; Second Trimester: Weeks 14 to 27; Third Trimester: Weeks 28 to 40+.
How long does a full-term pregnancy last?
A full-term pregnancy is considered 40 weeks (280 days) from the last period date.
How is a due date calculated?
By Naegele's rule: add 280 days - 40 weeks - to the first day of the last menstrual period, which assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles the estimate shifts accordingly, which is why a first-trimester ultrasound is more accurate and usually supersedes the LMP date.
Why is pregnancy dated from my last period rather than conception?
Because the date of the last menstrual period is something you can usually report, while the date of conception almost never is — ovulation is not directly observable and sperm survive up to five days. Dating from the LMP adds roughly two weeks of "pregnancy" before conception occurred, which is why you are considered four weeks pregnant about two weeks after conception.
How accurate is Naegele's rule?
It assumes a 28-day cycle with ovulation on day 14, so it is only as accurate as that assumption. For someone with a 35-day cycle it typically places the due date about a week early. A first-trimester ultrasound measuring crown-rump length dates the pregnancy to within about five days and is what clinicians use when the two disagree.
How likely am I to give birth on my due date?
About 4%. Roughly 80% of births occur between 37 and 42 weeks, and first pregnancies tend to run a few days longer than subsequent ones. The due date marks the centre of a distribution that is several weeks wide, not an appointment — which is worth remembering when it passes.
What do the trimesters actually mark?
First trimester is weeks 1 to 13, second 14 to 27, third 28 to birth. The divisions are conventional rather than biological, but they track real transitions: organ formation completes in the first, most growth happens in the second, and the third is dominated by weight gain and lung maturation. Miscarriage risk drops sharply at the end of the first.
What does 'viability' mean in this context?
The gestational age at which survival outside the womb becomes possible with intensive care — currently around 22 to 24 weeks, though outcomes at the earliest end remain poor and vary considerably by hospital. Survival rates climb steeply with each additional week, which is why delaying a preterm birth even briefly matters clinically.
How due dates are calculated
Naegele's rule: due date = LMP + 280 days (40 weeks) Cycle-adjusted: due date = LMP + 280 + (cycle length − 28) days
The 280 days assume ovulation on day 14 of a 28-day cycle followed by 266 days of gestation. A longer cycle means later ovulation, so the adjustment above shifts the estimate accordingly — a 32-day cycle pushes the due date four days later than the unadjusted rule would.
Why ultrasound usually wins
Early embryos grow at a remarkably consistent rate, so crown-rump length between 8 and 13 weeks dates a pregnancy to within about five days regardless of cycle irregularity or uncertain dates. Most guidelines redate the pregnancy when the scan and the LMP estimate differ by more than five to seven days in the first trimester. Later scans are much less reliable for dating, because growth variation between fetuses widens.
This is an estimate, not clinical care. Your midwife or doctor's dating supersedes any calculator. Seek prompt medical advice for bleeding, severe pain, reduced fetal movement or any concern about your pregnancy.