Gestational Age Calculator
Find how many weeks pregnant you are.
The date to measure to
Results update as you type.
This calculator provides estimates based on population research. Results are not medical advice. Consult a qualified healthcare provider before making health or nutrition decisions.
How it works
The gestational age calculator tells you how far along a pregnancy is, measured in completed weeks and days from the first day of the last menstrual period to a chosen reference date, usually today. Gestational age is the standard clinical way to describe pregnancy progress, and it underpins everything from scheduling scans and screening tests to assessing whether a baby is preterm, term, or overdue. Because dating conventionally begins at the last period rather than at conception, the gestational age is roughly two weeks greater than the embryo's true developmental age; this is simply the accepted convention that lets clinicians worldwide use a consistent timeline. The calculator also reports the trimester: the first trimester runs through the end of week 12, the second from weeks 13 to 27, and the third from week 28 to birth, which is expected around 40 weeks. Knowing the exact week helps expectant parents follow developmental milestones and understand what to expect at each prenatal visit. Keep in mind that an early ultrasound may adjust the dating if it differs from the last-period estimate, and that this tool provides general information rather than medical assessment. Enter the first day of your last period and the reference date to see how many weeks pregnant you are.
gestational age = (reference date − LMP), shown as completed weeks + days
Gestational age is counted from the first day of the last menstrual period to a reference date, giving a total number of days that is then expressed as completed weeks plus leftover days. Because dating starts at the last period rather than conception, the count is about two weeks ahead of the embryo's actual age. The trimester follows from the week count: first up to 13 weeks, second to 27, and third thereafter.
Source:Gestational age from LMP (ACOG)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter first day of last period and reference date (today).
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Completed weeks updates instantly as you type — no button to press.
Frequently asked questions
What is gestational age?
Gestational age is how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period. It is the standard measure clinicians use to track pregnancy, schedule tests, and judge whether a baby is preterm or full term. A full-term pregnancy is around 40 weeks of gestational age, measured from the last period rather than from conception.
Why is it counted from the last period?
Dating from the first day of the last menstrual period gives a reliable, known starting point, since most people can identify that date while the exact moment of conception is usually unknown. As a result, gestational age is about two weeks greater than the embryo's actual age from conception. This convention is used consistently worldwide so pregnancy progress can be compared and communicated clearly.
How do trimesters relate to gestational age?
Pregnancy is divided into three trimesters by gestational age. The first trimester runs from week 1 through the end of week 12, the second from week 13 to week 27, and the third from week 28 until birth. Each trimester marks different stages of fetal development and different considerations for prenatal care, which is why knowing your current week matters.
What is considered full term?
A pregnancy is generally considered full term between 37 and 42 weeks of gestational age. Birth before 37 weeks is preterm, and after 42 weeks is post-term. Most babies arrive around 40 weeks. Gestational age at delivery is important because it strongly influences a newborn's health and the care they may need, so accurate dating throughout pregnancy is valuable.
Can ultrasound change the gestational age?
Yes. An early ultrasound, especially in the first trimester, measures the embryo directly and can provide a more accurate gestational age than the last-period estimate, particularly for people with irregular cycles or uncertain period dates. If the ultrasound dating differs significantly, clinicians usually adjust the estimated due date and gestational age to the scan-based figure.