Pregnancy Calculator

Formula

How to use it

1. Choose what you know: your last period, a conception date or a due date.
2. Enter the date. For a last period, change the cycle length only if your cycles are reliably longer or shorter than 28 days.
3. Leave “How far along on” at today, or pick another date to see how far along you will be then.
4. Read your weeks and days, the week you are in, your trimester and the key dates; the calendar lists when each week begins.

Worked examples

How weeks of pregnancy are counted

Pregnancy is dated from the first day of the last menstrual period, about two weeks before conception, which is why a full-term pregnancy is called 40 weeks. Gestational age is written as completed weeks plus days: 12 weeks 6 days means 12 full weeks have passed and you are in your 13th week. That is the convention used by the World Health Organization and in pregnancy care.

If you have had a dating ultrasound, enter the due date it gave you. An early ultrasound measurement is more accurate than period dates, and ACOG recommends using it when the two disagree by more than a set amount for the stage of pregnancy. Your care team’s dates take priority over this calculator.

After IVF, the transfer date gives the most exact dating. This calculator has no IVF input: use the Due Date Calculator, which takes a transfer date and embryo age, or enter the due date your clinic gave you.

Trimester boundaries are a convention, not a biological event. This calculator uses the boundaries in ACOG’s guidance (the first trimester runs through 13 weeks 6 days). Some services, including the NHS, describe the first trimester as weeks 1 to 12, so a date near the boundary may be labeled differently there.

When to talk to a doctor or midwife

Contact your midwife, doctor or maternity unit promptly about bleeding from the vagina, severe or persistent tummy pain, fluid leaking from the vagina, a severe headache, changes in your vision, or sudden swelling of your face, hands or feet. If you think you are pregnant, contact a doctor or midwife to start your care; they will confirm your dates.

If your baby’s movements slow down or change after you have started to feel them, contact the maternity unit the same day. Do not wait until the next day.

In early pregnancy, one-sided pain low in the tummy, with or without bleeding, needs urgent care, because it can be a sign of an ectopic pregnancy.

From 41 weeks (late term) and 42 weeks (post-term) the result says so and counts the days past the due date. Follow your maternity team’s plan for monitoring or induction.

This is an estimate for general information, not medical advice. Talk to a doctor or other qualified clinician before making health decisions based on it.

Sources

Due date as 280 days after the first day of the last period, and the use of first-trimester ultrasound to confirm or change it: American College of Obstetricians and Gynecologists, Committee Opinion No. 700, “Methods for Estimating the Due Date” (2017, with AIUM and SMFM).

First trimester “up to and including 13 6/7 weeks”: ACOG Committee Opinion No. 700. Second and third trimesters from 14 0/7 and 28 0/7 weeks follow the same convention.

Full term from 39 weeks 0 days (early term 37 0/7–38 6/7, late term from 41 0/7, postterm from 42 0/7): ACOG Committee Opinion No. 579, “Definition of Term Pregnancy” (2013).

Gestational age measured from the first day of the last normal menstrual period, in completed weeks: World Health Organization, International Classification of Diseases (ICD-10), Volume 2, definitions for perinatal statistics.

Trimesters described as weeks 1–12, 13–27 and 28–40, and symptoms to report urgently: NHS, “Your pregnancy and baby guide” and “Vaginal bleeding in pregnancy” (nhs.uk).

Ectopic pregnancy and reduced baby movements: NHS, “Ectopic pregnancy” and “Your baby’s movements” (nhs.uk).