Pregnancy week calculator: how many weeks pregnant you are
This pregnancy week calculator gives gestational age today, in weeks and days, from the first day of the last period, the due date, a conception date, an ultrasound scan or an IVF transfer date.
Seek help now if you have any of these symptoms
Bleeding and cramping are common in early pregnancy and often do not mean miscarriage. The symptoms below, however, need to be assessed straight away.
- Soaking two or more sanitary pads per hour, or passing large clots
- Severe lower abdominal pain, especially on one side
- Pain at the tip of the shoulder
- Dizziness, fainting, or collapse
- Signs of shock — pale, clammy, rapid heartbeat, breathlessness
- Pain on opening bowels, or bladder or bowel changes
- Feeling very unwell, or feverish
Showing routes for United States.
A gestational age calculator counts from the last period, not from conception
Pregnancy is dated from the first day of the last menstrual period (LMP). The convention counts 280 days from that date and assumes a 28-day cycle with ovulation on day 14 (ACOG Committee Opinion 700, 2017, reaffirmed 2022). Conception therefore occurs about two weeks after the start of the count, so gestational age runs roughly two weeks ahead of the age of the embryo or fetus. Prenatal appointments, scan reports and due dates all use gestational age. The calculator gives both figures, labeled.
How far along you are depends on the date the count starts from
In the US, ACOG describes an ultrasound scan in the first trimester, up to and including 13 weeks and 6 days, as the most accurate way to establish or confirm gestational age; measurement of crown–rump length in that period is accurate to within 5 to 7 days. ACOG advises changing a last-period due date to the scan date if the two differ by more than 7 days, or by more than 5 days before 9 weeks (ACOG Committee Opinion 700, 2017, reaffirmed 2022). After IVF, ACOG assigns the due date from the age of the embryo and the date of transfer. In the UK, NICE recommends offering a scan between 11 weeks and 2 days and 14 weeks and 1 day to determine gestational age (NICE NG201, 2021).
Calculating the pregnancy week by due date reproduces the prenatal count
Where a due date has already been set, gestational age is today minus the due date, plus 280 days. Entering that due date gives the same count used at prenatal appointments. Calculating the pregnancy week by LMP repeats the original estimate, adjusted for cycle length.
The second trimester begins at 14 weeks and the third at 28
The first trimester runs to 13 weeks and 6 days and the second from 14 weeks to 27 weeks and 6 days (ACOG Committee Opinion 700); the third runs from 28 weeks until birth. The result gives the trimester alongside the week count.
Sources
- American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine and Society for Maternal-Fetal Medicine. Methods for estimating the due date (Committee Opinion No. 700). Obstetrics & Gynecology. 2017;129(5):e150–e154. doi:10.1097/AOG.0000000000002046. Reaffirmed 2022.
- National Institute for Health and Care Excellence. Antenatal care (NG201). Published 19 August 2021; last reviewed 27 December 2024.