Understanding your gestational age result
Trimesters group the pregnancy into three stages, and ACOG defines term categories for the weeks around the due date. Both are shown below using gestational age in weeks and days.
| Stage | Gestational age | Notes |
|---|---|---|
| First trimester | 0w0d – 13w6d | Dating is most accurate in this window; first-trimester ultrasound is the reference method |
| Second trimester | 14w0d – 27w6d | Routine anatomy ultrasound is commonly performed around 18–22 weeks |
| Third trimester | 28w0d – birth | Visit frequency typically increases as the due date approaches |
| Early term (ACOG) | 37w0d – 38w6d | ACOG/SMFM term definitions, 2013 |
| Full term (ACOG) | 39w0d – 40w6d | The estimated due date falls at 40w0d |
| Late term / postterm (ACOG) | 41w0d – 41w6d / 42w0d+ | Care teams monitor pregnancies that continue past the due date |
- Only about 4–5% of babies arrive on their exact due date; the EDD is a planning landmark, not a prediction of the birth day.
- LMP-based dating assumes a regular cycle with ovulation near day 14; irregular cycles, recent hormonal contraception or uncertain period dates make ultrasound dating especially valuable.
- If your ultrasound-assigned due date differs from this calculator's estimate, the ultrasound dating takes precedence — that is the ACOG-recommended standard.
- This tool is educational. Decisions about tests, scans and care timing belong with your OB-GYN or midwife, who will interpret dating alongside your full clinical picture.
What is gestational age?
Gestational age is the standard clinical measure of how far a pregnancy has progressed. It is counted from the first day of the last menstrual period (LMP), not from conception, and is expressed in completed weeks plus days — for example, "12 weeks 3 days" (often written 12w 3d). Because ovulation typically occurs about two weeks after the LMP, gestational age runs roughly two weeks ahead of the embryo's actual age. A pregnancy dated this way reaches its estimated due date at 40 weeks 0 days.
The American College of Obstetricians and Gynecologists (ACOG), together with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, recommends ultrasound measurement of the embryo in the first trimester (up to 13 weeks 6 days) as the most accurate method to establish or confirm gestational age. When the menstrual-dating estimate differs from the first-trimester ultrasound estimate by more than about 7 days, guidance is to redate the pregnancy using the ultrasound (ACOG Committee Opinion 700, 2017). A calendar tool like this one is a helpful orientation, and your OB-GYN or midwife will confirm the official dating.
Knowing gestational age matters because prenatal care is organized around it: screening tests, scans and check-ups are scheduled at particular weeks, and ACOG defines term categories by gestational age — early term (37w0d–38w6d), full term (39w0d–40w6d), late term (41w0d–41w6d) and postterm (42w0d and beyond). Every pregnancy progresses a little differently, and your care team is the right place to bring any questions about your dates.
How to use this gestational age calculator
- Choose your starting point. Select "Last menstrual period" if you know the first day of your last period, or "Known due date" if a due date has already been given to you — for example at an ultrasound appointment.
- Enter the date. For the LMP method, enter the first day of your most recent period before pregnancy; for the due-date method, enter the estimated due date you were given.
- Read your gestational age in weeks and days, your current trimester, the estimated due date, and the approximate days remaining — results update instantly.
- Bring the result to your OB-GYN or midwife as a conversation starter; a first-trimester ultrasound is the most accurate way to confirm dating.
How gestational age is calculated
From an LMP date, gestational age is the number of days elapsed since the first day of that period, expressed as completed weeks and days. The estimated due date (EDD) is the LMP plus 280 days (40 weeks) — the convention known as Naegele's rule. From a known due date, the calculator reverses the same arithmetic: it subtracts 280 days from the EDD to recover the dating start point, then counts forward to today.
Trimester boundaries used here follow common clinical convention: the first trimester runs to 13 weeks 6 days, the second trimester from 14 weeks 0 days, and the third trimester from 28 weeks 0 days. Sources vary slightly on the exact boundaries — some place the third trimester at 27 weeks — because trimesters are descriptive groupings rather than precise clinical thresholds.
Common mistakes
- Counting from the estimated conception date instead of the last menstrual period — gestational age is defined from the LMP and runs about two weeks ahead of embryonic age.
- Entering the last day of the period rather than the first day, which shifts the estimate by several days.
- Treating the due date as a deadline; a full-term window spans several weeks, and only a small minority of births occur on the EDD itself.
- Relying on calendar dating despite irregular cycles — in that case first-trimester ultrasound dating is considerably more reliable.
- Using this calculator to time or interpret medical tests on your own; screening schedules are set by your care team based on confirmed dating.
Pertanyaan yang sering diajukan
How many weeks pregnant am I?
Gestational age is counted from the first day of your last menstrual period (LMP). Divide the days elapsed since that date by seven: the whole number is your completed weeks and the remainder is the extra days. For example, 87 days after the LMP corresponds to 12 weeks 3 days. If you only know your due date, subtracting 280 days from it recovers the LMP-equivalent starting point.
Why is gestational age counted from the last period and not from conception?
The first day of the last menstrual period is usually a known, recordable date, whereas the exact day of conception is rarely known. Clinical convention therefore dates pregnancy from the LMP, which places conception at roughly week 2 of gestational age. This is why a pregnancy is "4 weeks" at around the time of a missed period even though fertilization occurred about two weeks earlier.
What is the most accurate way to determine gestational age?
An ultrasound measurement of the embryo's crown-rump length in the first trimester (up to 13 weeks 6 days) is the most accurate method, according to ACOG Committee Opinion 700. If the menstrual-history estimate and the first-trimester ultrasound estimate differ by more than about 7 days, the pregnancy is redated to the ultrasound estimate. Accuracy of ultrasound dating decreases in the second and third trimesters.
When does each trimester start and end?
By the convention used in this calculator, the first trimester spans 0 to 13 weeks 6 days of gestational age, the second trimester spans 14 weeks 0 days to 27 weeks 6 days, and the third trimester runs from 28 weeks 0 days until birth. Some sources start the third trimester at 27 weeks; trimester boundaries are descriptive groupings rather than exact clinical cut-offs.
Will my baby arrive on the estimated due date?
Probably not on the exact date — only around 4–5% of babies are born on their EDD. The due date marks 40 weeks 0 days of gestational age and serves as a planning landmark. ACOG defines full term as 39 weeks 0 days to 40 weeks 6 days, and many healthy births occur across the surrounding weeks. Your care team monitors pregnancies that continue past the due date.
What if my cycle is irregular — can I still use LMP dating?
Calendar-based dating assumes ovulation about 14 days after the period starts, so irregular or long cycles reduce its accuracy. In that situation a first-trimester ultrasound is the reliable way to establish gestational age, and it is the method ACOG recommends for confirming or redating any pregnancy. Share your cycle history with your OB-GYN or midwife so dating can be personalized.
Referensi
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology 2017; 129(5): e150–e154.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 579: Definition of Term Pregnancy. Obstetrics & Gynecology 2013; 122(5): 1139–1140.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (patient education). acog.org.
- NHS. Pregnancy week-by-week — dating a pregnancy from the last menstrual period. nhs.uk.
- Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Human Reproduction 2013; 28(10): 2848–2855.