Pregnancy Calculator
Enter the first day of your last menstrual period (LMP) to estimate your due date and current progress.
This is an estimate only — based on the standard 40-week (280-day) method. Always confirm with your healthcare provider.
What Is a Pregnancy Due Date Calculator?
Among the first questions of every pregnancy is "when is the baby due?" This calculator answers it with the standard obstetric method: enter the first day of your last menstrual period (LMP), and it counts forward the 280 days — exactly 40 weeks — that define a full-term pregnancy, also showing which trimester you are in. It is the same arithmetic your midwife or obstetrician performs at a first appointment, before any ultrasound refinement.
Counting from the last period surprises many people: by this convention you are already about two weeks "pregnant" at conception and roughly four weeks along when a home test turns positive. The convention survives because the LMP is a date most women know, while the moment of conception usually is not. Gestational age — weeks since the LMP — is the language of every scan, test, and milestone that follows.
How to Use the Pregnancy Calculator
- Enter the first day of your last menstrual period — the first day of bleeding, not the day it ended.
- Click Calculate.
- See your estimated due date and your current trimester.
- Share the estimate at your first prenatal appointment, where an early ultrasound may adjust it.
Naegele's Rule and the Three Trimesters
The 280-day count is formalized as Naegele's rule: take the LMP, add one year, subtract three months, and add seven days. Pregnancy is then divided into trimesters — the first running to about week 13, when organs form and screening tests begin; the second, weeks 14–27, typically the most comfortable, when anatomy scans happen and movement is felt; and the third, from week 28 to birth, when the baby gains most of its weight. Only about 1 in 20 babies arrive on the exact due date; the normal window for a healthy birth spans roughly weeks 37 to 42.
Good to Know in Early Pregnancy
- An early dating ultrasound (around weeks 8–13) is the most accurate way to set a due date and supersedes the LMP estimate if they disagree by more than a few days.
- If your cycles are much longer or shorter than 28 days, or irregular, mention it — the LMP method assumes ovulation on day 14.
- Folic acid (400–800 micrograms daily) is recommended before conception and through the first trimester to reduce neural tube defects — ask your clinician about your situation.
How accurate is a due date calculator?
The LMP method assumes a 28-day cycle with ovulation on day 14, so it is a solid estimate for regular cycles and a rougher one otherwise. Even a perfectly calculated due date is a center point, not an appointment: most babies arrive within about two weeks on either side, and first-time mothers deliver after the due date more often than before it.
I don't know my last period date. What then?
Use your best estimate here for a rough date, but rely on an early ultrasound for the real one. Crown-rump length in the first trimester dates a pregnancy to within about five to seven days, which is why clinicians order a dating scan when the LMP is unknown, uncertain, or inconsistent with the examination.
Can my due date change later in pregnancy?
It can be revised after the first-trimester ultrasound, and after that it generally should not change: later ultrasounds measure size, which varies naturally between babies, rather than age. A due date set early is kept even if third-trimester scans suggest a bigger or smaller baby — that difference is growth, not dates.
What if I conceived through IVF?
IVF pregnancies are dated from the transfer and egg retrieval dates, which are more precise than any LMP calculation. Your fertility clinic will give you a due date based on the embryo's age at transfer. The LMP method here does not apply, though the trimester milestones after dating are the same.