Gestational age is calculated from the first day of your last menstrual period (LMP). Count the elapsed days from that date to today and divide by seven; the quotient is completed weeks and the remainder is days. The American College of Obstetricians and Gynecologists (ACOG) notes pregnancy lasts about 40 weeks from LMP, and a dating ultrasound around week 12 refines accuracy.
How to calculate gestational age
Clinical dating begins from a single reference date: the first day of your last normal menstrual period (LMP). The UK National Health Service (NHS) calculates estimated due date (EDD) from this date, noting that pregnancy duration is clinically timed from your LMP.
To calculate manually, count the total days between your LMP and today, then divide by seven. The whole number gives completed gestational weeks, and the remainder gives additional days. The American College of Obstetricians and Gynecologists (ACOG) explains that gestational age is expressed in weeks plus days, such as "36 weeks and 3 days."
For example:
- Your last menstrual period began on March 1, and today is April 20.
- March has 30 remaining days, plus 20 days in April, giving 50 total elapsed days.
- Dividing 50 by 7 gives 7 weeks with a remainder of 1 day (7 weeks and 1 day pregnant).
Checking the day count between dates in your calendar produces the identical mathematical result.
Why is gestational age timed from the first day of your period?
The exact day of conception is rarely known with certainty, whereas the first day of menstruation is an identifiable calendar event. Medical dating therefore starts from that day.
ACOG explains that conception is assumed to happen about two weeks after the first day of your last period. That is why the count starts before you are actually pregnant, and why the first two weeks are part of the 40.
The NHS points out that this two week assumption is based on a 28 day cycle and does not apply to shorter or longer menstrual cycles. Online due date tools adjust their estimates if your typical cycle length deviates from 28 days.
Calculating your estimated due date
According to ACOG, full term gestation lasts approximately 40 weeks (280 days) from your LMP. Adding 280 days to the first day of your last period gives your estimated due date. In the March 1 example, this projects a due date of December 6.
This date represents an estimate. The NHS notes that most spontaneous births occur between 37 and 42 weeks of gestation. ACOG reports that only about 1 in 20 women deliver exactly on their estimated due date. Even so, the estimated due date is clinically vital: ACOG uses it to track fetal growth, evaluate development, and schedule routine prenatal screenings.
To see how gestational weeks translate into calendar months and trimesters, read pregnancy weeks to months.
How ultrasound refines gestational age
If you do not know your LMP or have irregular cycles, ultrasound dating provides clarity. According to the NHS, a dating ultrasound scan is typically performed between 10 and 14 weeks of gestation. This scan determines gestational age, establishes the due date, checks for multiple pregnancies, and verifies that the embryo is developing normally in the uterus. The NHS notes that ultrasound dating is more accurate than menstrual dates alone.
ACOG explains that first trimester ultrasound measures fetal crown-rump length (CRL), calculating gestational age via standardized biometric formulas. In IVF pregnancies, the due date is established using embryo age and transfer date.
If your ultrasound date differs slightly from your LMP calculation, follow the date confirmed by your doctor or midwife.
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When to seek medical advice
The NHS recommends contacting a midwife or doctor as soon as you confirm pregnancy, or if you are unsure of your last period date.
The NHS notes that light spotting can happen in early pregnancy, but vaginal bleeding should always be evaluated by a healthcare professional. Call emergency services immediately if bleeding is accompanied by:
- Severe, sharp, or persistent lower abdominal pain
- Shoulder tip pain
- Dizziness, faintness, nausea, or loss of consciousness
- Heavy bleeding that rapidly soaks through a sanitary pad
In an emergency, call 911 in the US, 999 in the UK, or 112 in Europe.
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Frequently asked questions
When does pregnancy calculation begin?
Clinical dating starts from the first day of your last menstrual period. According to ACOG, this date occurs roughly two weeks prior to conception and is counted as part of the 40 week pregnancy.
Does a longer menstrual cycle change the due date?
Yes. The standard count assumes a 28 day cycle, so the NHS due date calculator asks for your cycle length if yours is shorter or longer. A first trimester dating ultrasound provides the most reliable timeline if your cycles are irregular.
What symptoms are common in early pregnancy?
The NHS notes that early pregnancy can be symptom free, or accompanied by nausea, tiredness, breast tenderness, and frequent urination. For dietary self-care tips, see our guide on pregnancy what helps with nausea.
Sources
- NHS: Due date calculator
- NHS: 12-week scan
- NHS Best Start in Life: Week 4
- NHS: Periods and fertility in the menstrual cycle
- NHS: Vaginal bleeding in pregnancy
- ACOG: How your fetus grows during pregnancy
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