Estimated gestational age: how pregnancy weeks are counted

Gestational age is counted from the first day of your last period

The American College of Obstetricians and Gynecologists (ACOG) explains in its patient FAQ "How Your Fetus Grows During Pregnancy" that a normal pregnancy lasts about 40 weeks from the first day of the last menstrual period (LMP). Pregnancy is assumed to start 2 weeks after that day, so an extra 2 weeks is counted at the beginning, when you are not yet pregnant (ACOG, 2024). ACOG defines gestational age as how far along a woman is in her pregnancy, usually reported in weeks and days, and writes 36 weeks and 3 days as "36 and 3/7 weeks" (ACOG, 2024). The NHS also dates a pregnancy from the first day of the last period (NHS, 2023).

The count starts before conception because the day of conception is rarely known

The March of Dimes explains that conception usually happens about 2 weeks after the start of the last menstrual period, and that you may not know the exact day you become pregnant. This is why health care providers use the last period to find out how far along you are (March of Dimes, 2023). For the same reason, a calculator that asks for the first day of your last period, such as the pregnancy week calculator on this site, is counting the same way a clinic does.

Gestational age is about two weeks more than the age of the embryo or fetus

ACOG's glossary defines an embryo as the stage of development from fertilization up to 8 weeks, and the fetus as the stage beyond 8 completed weeks after fertilization (ACOG, 2024). Counted from fertilization, therefore, the age of the embryo or fetus is about two weeks less than the gestational age, the two weeks that ACOG describes as counted before pregnancy begins. A pregnancy that is 6 weeks by gestational age is about 4 weeks from fertilization, on ACOG's assumption that ovulation falls about two weeks after the last period starts. Both figures are correct; they count from different starting points, and clinicians, midwives and the calculators on this site use gestational age.

Estimated means the date it rests on can be wrong

ACOG Committee Opinion 700, "Methods for Estimating the Due Date" (2017), written with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, states that by convention the due date is 280 days after the first day of the last period. That convention assumes a regular 28-day cycle with ovulation on day 14, so it does not account for inaccurate recall of the date, irregular cycles or variable timing of ovulation. The opinion reports that approximately one half of women accurately recall their LMP (ACOG, 2017). Gestational age is therefore set from the LMP, from the first accurate ultrasound examination, or from both, and ACOG says ultrasound measurement in the first trimester, up to and including 13 weeks and 6 days, is the most accurate method (ACOG, 2017).

After IVF, the gestational age is not taken from the last period. ACOG's opinion says the age of the embryo and the date of transfer assign it (ACOG, 2017). The IVF due date calculator applies that rule.

A pregnancy with no early scan is called suboptimally dated

ACOG's term for a pregnancy that has no ultrasound examination confirming or revising the due date before 22 weeks and 0 days is "suboptimally dated" (ACOG, 2017). The term describes how well the date is known, and it says nothing about the pregnancy itself. Due date based on ultrasound explains when a scan moves the date, and estimated due date covers how accurate the date is.

How the UK and the US write gestational age

Both count from the last period in weeks and days. The style differs: NICE writes 11 weeks and 2 days as 11+2 (NICE, 2021), while ACOG writes 13 6/7 weeks or 13 weeks and 6 days (ACOG, 2017). The NHS says pregnancy normally lasts from 37 to 42 weeks from the first day of the last period (NHS, 2024). Pregnancy weeks to months shows where ACOG, the NHS and the US Office on Women's Health place the trimester lines.

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.