Weekly tracking
Gestational age: decode weeks and trimesters
Calculate the exact week of pregnancy and learn which medical checks align with each stage.
Free online tool
Gestational age
What gestational age means
Gestational age counts the number of weeks that have passed since the first day of the mother's last menstrual period (LMP). This dating system is the global standard used by obstetricians, midwives, and prenatal guidelines published by the American College of Obstetricians and Gynecologists (ACOG) and the World Health Organization (WHO). It differs from fetal age, sometimes called conceptional age, which starts roughly two weeks later at the moment of fertilization.
Understanding which week you are in is not just trivia. Every prenatal blood test, ultrasound, and vaccination has a recommended window tied to gestational age. Knowing your exact week helps you prepare for each appointment, understand test results, and recognize when something falls outside the expected timeline.
How gestational age is calculated
The simplest method is to count forward from the first day of your LMP. If your cycles are regular at around 28 days, ovulation likely occurred near day 14, and the embryo implanted a few days later. Still, the count begins at the LMP, meaning you are technically 'two weeks pregnant' at the time of conception.
When cycles are irregular, or the LMP date is uncertain, an early ultrasound between 8 and 13 weeks provides a more accurate estimate. The sonographer measures the crown-rump length (CRL) of the embryo and compares it to standardized growth charts. According to ACOG Committee Opinion 700 (2017), if the ultrasound-based date differs from the LMP-based date by more than five days in the first trimester, the ultrasound date should be used.
For pregnancies achieved through in vitro fertilization (IVF), the calculation uses the embryo transfer date. A day-5 blastocyst transfer, for example, adds 19 days (14 days plus 5) to derive the equivalent LMP date.
Trimester breakdown: what happens when
Trimester snapshot
Weeks 1-12
First trimester
All major organs begin to form. The heart starts beating around week 6. Nausea peaks between weeks 8 and 10. Key tests include dating ultrasound, first-trimester screening for chromosomal conditions, and baseline blood work.
Weeks 13-27
Second trimester
The baby grows rapidly and begins to move. The anatomy scan at 18-22 weeks checks organ structure. Glucose screening for gestational diabetes is typically performed between weeks 24 and 28.
Weeks 28-40
Third trimester
Lungs mature, fat stores develop, and the baby gains most of its birth weight. Growth scans may be ordered if the fundal height is off track. Group B Streptococcus (GBS) screening occurs at 36-37 weeks.
Week-by-week developmental milestones
Weeks 4-8: Foundation building
By week 4, the embryo has implanted in the uterine wall and the placenta begins to form. The neural tube, which will become the brain and spinal cord, closes by week 6. The heart has developed four chambers and starts pumping blood. Tiny limb buds appear. This is why folic acid supplementation (400-800 micrograms daily) is critical before conception and through the first trimester: it reduces neural tube defects by up to 70%, according to data from the CDC.
Weeks 9-12: Transition to fetus
At week 9, the embryo is officially called a fetus. Fingers and toes separate, and bones begin to harden. By week 12, the fetus is about 6 centimeters long and can make sucking motions. The risk of miscarriage drops significantly after a heartbeat is confirmed at the end of the first trimester. First-trimester combined screening (nuchal translucency ultrasound plus blood markers) is usually performed between weeks 11 and 14.
Weeks 13-20: Movement and anatomy
Many mothers feel the first flutters of movement, called quickening, between weeks 16 and 20. By week 18, the baby can hear sounds. The detailed anatomy scan at 18-22 weeks examines the brain, heart, kidneys, spine, and limbs. Sex can usually be determined at this time if parents wish to know.
Weeks 21-27: Viability threshold
At 24 weeks, the fetus reaches a critical viability milestone. Survival outside the womb, while requiring intensive care, becomes possible. The lungs begin producing surfactant, a substance that prevents air sacs from collapsing. The eyes can open and close. A glucose tolerance test is usually performed between weeks 24 and 28 to screen for gestational diabetes, which affects roughly 6-9% of pregnancies according to the CDC.
Weeks 28-36: Rapid growth
The baby gains about 200 grams per week during this phase. Fat deposits smooth out the skin and help regulate body temperature after birth. Brain development accelerates dramatically. If preterm birth is a concern, corticosteroid injections can be given between 24 and 34 weeks to speed up lung maturation.
Weeks 37-40: Full term
ACOG defines early term as 37-38 weeks, full term as 39-40 weeks, late term as 41 weeks, and post-term as 42 weeks or beyond. The baby is typically head-down by week 37. Labor can begin naturally at any point within this range. Induction is generally considered after 41 weeks to reduce the risk of stillbirth and placental insufficiency.
Using the gestational age calculator
Our calculator takes your LMP or the gestational age from a recent ultrasound and returns the exact week and day, elapsed days, weeks remaining, and your current trimester. Here is how to get the most from it:
- Enter the first day of your last menstrual period or the date provided by your ultrasound.
- Double check the date format (day/month/year or month/day/year depending on your locale).
- The tool will display the current gestational week plus the number of days remaining until 40 weeks.
- Update the value after each prenatal visit to keep a clear log of your pregnancy timeline.
- Note relevant symptoms per week so you can discuss them with your care provider at the next appointment.
When gestational age matters most
Certain medical decisions are tightly linked to gestational age. Antenatal corticosteroids for lung maturity are only effective within a specific window. The timing of cervical cerclage, progesterone supplementation, and Rh immunoglobulin injection all depend on accurate dating. If your LMP and ultrasound dates disagree, work with your provider to establish the best estimated due date and use it consistently throughout the pregnancy.
Red flags at any gestational age
While every pregnancy is unique, certain symptoms warrant immediate medical evaluation regardless of the week:
- Vaginal bleeding, especially if heavy or accompanied by cramping.
- Severe or persistent abdominal pain that does not resolve with rest.
- Sudden swelling of the face or hands, severe headache, or vision changes (possible preeclampsia).
- Leaking of fluid from the vagina, which may indicate premature rupture of membranes.
- Noticeable decrease in fetal movements after 28 weeks (fewer than 10 movements in two hours).
- Fever above 38 degrees Celsius (100.4 degrees Fahrenheit) that does not respond to acetaminophen.
Seek care immediately
Severe abdominal pain, vaginal bleeding, or prolonged absence of fetal movements need urgent evaluation regardless of the week. When in doubt, call your maternity unit or go to your nearest emergency department.
Frequently asked questions
Is gestational age the same as fetal age?
No. Gestational age starts from the first day of the last menstrual period, while fetal age (conceptional age) starts from the estimated date of fertilization, roughly two weeks later. Medical records and due dates always use gestational age.
Can my due date change?
Yes. If a first-trimester ultrasound shows a significant discrepancy with the LMP-based date, your provider may adjust the estimated due date. Once established, however, the due date should not be changed based on later ultrasounds because growth variation increases as pregnancy progresses.
What if I do not know my LMP?
An early ultrasound is the next best option. Crown-rump length measurement in the first trimester can estimate gestational age with an accuracy of plus or minus five days.
This article is for educational purposes only and does not replace professional medical advice. Always consult your obstetrician or midwife for personalized prenatal care.
Sources
