Gestational age is how far along a pregnancy is, counted in weeks and days from the first day of your last menstrual period (LMP). By this count a full-term pregnancy lasts about 40 weeks (280 days), even though conception usually happens around week 2. Your provider may adjust the date using an early ultrasound or, after IVF, the embryo transfer date.
Key Takeaways
- Gestational age starts on the first day of your last period, not on the day you conceived.
- Fetal age (also called conception age) is usually about two weeks less than gestational age.
- Doctors confirm or correct dates with ultrasound, and an early scan is the most accurate dating tool.
- Gestational age sets the timing of screening tests, growth checks and the definition of term, preterm and postterm birth.
What Is Gestational Age?
Gestational age is the standard clock of pregnancy. It is the time elapsed since the first day of the last menstrual period, and it is the number your doctor, midwife and ultrasound report refer to when they say you are “10 weeks” or “28 weeks” pregnant. It is usually written as weeks plus days, so 10w3d means 10 weeks and 3 days.
Because ovulation and conception normally happen about two weeks after the period begins, you are already counted as “2 weeks pregnant” on the day you conceive. A typical pregnancy is therefore about 38 weeks from conception but 40 weeks by gestational age. You can see your own number with our gestational age calculator.
How Gestational Age Is Calculated
1. From your last menstrual period (LMP)
The traditional method is Naegele’s rule: take the first day of your last period, add 7 days, subtract 3 months, and you have an estimated due date. That is the same as adding 280 days to the LMP. The rule assumes a regular 28-day cycle with ovulation on day 14, so it can be off if your cycles are shorter, longer or irregular. A longer cycle means ovulation, and therefore conception, happened later than the rule assumes. Try the due date calculator to see the estimate for your dates, or read the due date guide for the full method.
2. From an ultrasound
An ultrasound measures the embryo or fetus and compares it with standard growth charts. In the first trimester the crown-rump length (CRL), the distance from head to bottom, is the most accurate measurement, typically within about 5 to 7 days. Later in pregnancy providers combine head circumference, abdominal circumference, biparietal diameter and femur length, and the margin of error grows with each trimester. This is why dating is best confirmed early. We explain the process step by step in how doctors calculate gestational age.
3. After IVF or other assisted conception
When the conception date is known exactly, dating works from the embryo’s age instead of the LMP. Gestational age on the day of transfer is the embryo’s age plus 14 days, so a day-5 blastocyst transferred today is 2 weeks and 5 days. See our IVF due date calculator and the guide to traditional vs. IVF due dates.
When Does a Due Date Get Changed?
Under guidance from the American College of Obstetricians and Gynecologists (ACOG), the LMP date is usually kept if it agrees closely with the ultrasound. If the two differ by more than the allowed margin for that stage of pregnancy, the due date is moved to the ultrasound date. The margin widens as the pregnancy progresses because scans are less precise later on.
| Gestational age at first ultrasound | Change the due date if LMP and ultrasound differ by more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days and later | 21 days |
Source: ACOG Committee Opinion No. 700, Methods for Estimating the Due Date. A pregnancy whose dating has not been confirmed by ultrasound before 22 weeks is considered “suboptimally dated”. Learn more about when gestational age gets adjusted. Once a due date is set by ultrasound it is generally not changed again.
Gestational Age vs. Fetal Age
| Gestational age | Fetal (conception) age | |
|---|---|---|
| Counted from | First day of last period | Day of conception |
| Typical full-term length | About 40 weeks | About 38 weeks |
| Used by | Doctors, ultrasound reports, due dates | Developmental and embryology references |
| Difference | Roughly 2 weeks higher | Roughly 2 weeks lower |
If you are 12 weeks pregnant by gestational age, the baby’s developmental age is closer to 10 weeks. When someone says “weeks pregnant” they almost always mean gestational age. The two are compared in detail in gestational age vs. fetal age.
How Doctors Use Gestational Age
Almost every prenatal decision is tied to gestational age. Providers use it to:
- Time screening and tests. For example the nuchal translucency scan around 11 to 13 weeks 6 days, the detailed anatomy scan around 18 to 22 weeks, and gestational diabetes screening around 24 to 28 weeks. Exact timing varies by provider; see prenatal screening tests.
- Interpret growth. Fetal measurements and weight are compared with what is expected for that exact week. Our fetal weight percentile calculator shows how this works.
- Plan delivery. Decisions about induction, monitoring and birth setting depend on whether the pregnancy is preterm, term or past due.
- Guide care after birth. Newborn care and the “corrected age” used for premature babies are based on gestational age at birth.
Pregnancy is also divided into trimesters by gestational age: the first trimester runs to about 13 weeks 6 days, the second trimester from 14 weeks to 27 weeks 6 days, and the third trimester from 28 weeks until birth.
Preterm, Full Term and Postterm: What the Weeks Mean
| Category | Gestational age at birth |
|---|---|
| Preterm | Before 37 weeks 0 days |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Postterm | 42 weeks 0 days and later |
These are the definitions used by ACOG and the March of Dimes. They exist because babies born at 39 to 40 weeks generally have the best outcomes, and every extra week in the womb matters. Look up unfamiliar words in our pregnancy terms glossary.
How Accurate Is Gestational Age?
Gestational age is an estimate, not a fixed fact. Your due date is the midpoint of a window rather than a deadline, and only around 1 in 20 babies arrives on the exact estimated date. Most healthy births occur between 37 and 42 weeks. Dating is most reliable when it is based on a known, regular LMP confirmed by a first-trimester scan, or on an IVF transfer date. Dating is least reliable when the LMP is uncertain and the first scan happens late.
Frequently Asked Questions
What is gestational age in simple terms?
It is the number of weeks and days since the first day of your last menstrual period. It is the standard way pregnancy length is measured.
Why do doctors count from the last period instead of conception?
Most people remember the date their period started but cannot know exactly when conception happened. The LMP is a clear starting point that works for almost everyone, and ultrasound can refine it.
Is gestational age the same as how many weeks pregnant I am?
Yes. “Weeks pregnant” in medical settings means gestational age. The baby’s developmental (fetal) age is about two weeks less.
How many weeks is a normal pregnancy?
A full-term pregnancy is 39 weeks 0 days to 40 weeks 6 days. The average length counted from the LMP is 40 weeks, or 280 days.
Can gestational age change after the first ultrasound?
Yes, if the ultrasound and LMP dates differ by more than the ACOG margin for that stage (see the table above). Once the due date is revised or confirmed, it is generally kept for the rest of the pregnancy, even if later scans suggest a slightly bigger or smaller baby.
How is gestational age calculated with IVF?
It is based on the embryo’s age at transfer plus 14 days, not on the LMP. A day-5 embryo transfer corresponds to 2 weeks 5 days on the day of transfer.
How do I calculate my gestational age?
Count the weeks and days from the first day of your last period to today, or enter that date into the gestational age calculator. Your provider’s dating from ultrasound takes priority over a self-calculated date.
More answers are in our gestational age FAQs.
Explore More on Gestational Age
How doctors calculate it · When it gets adjusted · Milestones by gestational age · Pregnancy week by week · FAQs
References
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017.
- StatPearls. Pregnancy Dating. National Library of Medicine.
- StatPearls. Sonography Assessment of Gestational Age. National Library of Medicine.
- March of Dimes. What is full term?
Our content is written to the standards in our editorial policy and medical review process.
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.
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