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Calculate your pregnancy gestational age, due date, and current trimester based on your last menstrual period or ultrasound measurements.
Gestational age is the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks and days, starting from the first day of the woman's last menstrual period (LMP). This dating method is used universally in obstetrics because it provides a standardized reference point, even though conception typically occurs about two weeks after the LMP. Understanding your gestational age is crucial for monitoring fetal development, scheduling prenatal tests, and determining your due date.
It's important to distinguish between gestational age and fetal age. Gestational age counts from the LMP, while fetal age (or conceptional age) counts from the actual conception date, which is typically about 14 days after the LMP. Healthcare providers use gestational age because the exact conception date is often unknown. A full-term pregnancy is considered 40 weeks of gestational age, which is approximately 38 weeks of fetal age. Ultrasound measurements in early pregnancy can provide more accurate gestational age dating, especially if menstrual cycles are irregular.
The most common method for calculating gestational age is based on the first day of your last menstrual period (LMP). To calculate, count the number of weeks and days from the first day of your LMP to today's date. For example, if your LMP was 15 weeks and 3 days ago, your gestational age is 15 weeks 3 days. The due date is calculated by adding 280 days (40 weeks) to the first day of the LMP. This method assumes a regular 28-day menstrual cycle with ovulation occurring on day 14.
Ultrasound dating is considered more accurate, especially when performed in the first trimester (before 13 weeks). During an ultrasound, measurements such as crown-rump length (CRL) in early pregnancy or biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) in later pregnancy are used to estimate gestational age. If there's a discrepancy of more than 7 days between LMP dating and ultrasound dating in the first trimester, or more than 10-14 days in the second trimester, healthcare providers typically use the ultrasound date as it's more reliable.
Your pregnancy is divided into three trimesters: the first trimester (weeks 1-12), second trimester (weeks 13-26), and third trimester (weeks 27-40+). Each trimester brings different developmental milestones and screening tests. A pregnancy is considered full-term between 37 and 42 weeks. Babies born before 37 weeks are preterm, while those born after 42 weeks are post-term. Regular prenatal care uses gestational age to monitor appropriate fetal growth and development throughout your pregnancy journey.
| Weeks | Trimester | Key Milestones | Average Size |
|---|---|---|---|
| 4-6 | First | Heart begins beating | Poppy seed to lentil |
| 8-10 | First | Major organs forming | Raspberry to strawberry |
| 12 | First | End of first trimester | Lime (2.5 inches) |
| 13-16 | Second | Gender may be visible | Lemon to avocado |
| 20 | Second | Halfway point, anatomy scan | Banana (10 inches) |
| 24-26 | Second | Viability threshold | Ear of corn |
| 27-32 | Third | Rapid brain development | Squash to coconut |
| 37-40 | Third | Full term | Pumpkin (19-21 inches) |
Take prenatal vitamins with folic acid, stay hydrated, eat small frequent meals to manage nausea, and get plenty of rest. Schedule your first prenatal appointment.
Continue healthy eating, start gentle exercise if approved by your doctor, schedule your anatomy scan around week 20, and begin preparing your birth plan.
Count fetal movements daily, attend childbirth classes, prepare your hospital bag, discuss birth preferences with your healthcare provider, and watch for signs of labor.
Gestational age is calculated from the first day of your last menstrual period and is about 2 weeks longer than fetal age, which is calculated from conception. Healthcare providers use gestational age as the standard measurement.
Ultrasound measurements, especially in the first trimester, directly measure fetal development rather than relying on menstrual cycle assumptions. This is particularly helpful for women with irregular cycles or uncertain LMP dates.
Your due date may be adjusted based on early ultrasound measurements if there's a significant difference from LMP dating. However, due dates are rarely changed after the first trimester unless there's a clear medical reason.
LMP-based dating is accurate within 1-2 weeks if you have regular cycles. First-trimester ultrasound is accurate within 5-7 days. Only about 5% of babies are born on their exact due date, with most arriving within 2 weeks before or after.
If you're unsure of your LMP or have irregular cycles, an early ultrasound (ideally before 13 weeks) is the best method for establishing accurate gestational age and due date. Discuss this with your healthcare provider at your first prenatal visit.
A full-term pregnancy is 37-42 weeks of gestational age. Early term is 37-38 weeks, full term is 39-40 weeks, late term is 41 weeks, and post-term is 42+ weeks. The goal is delivery between 39-40 weeks for optimal outcomes.
The first ultrasound is typically performed between 8-14 weeks to confirm pregnancy, establish accurate dating, check for multiples, and ensure the pregnancy is developing in the uterus. Your healthcare provider will recommend the best timing for you.
No, while 40 weeks is the average, normal pregnancies can range from 37-42 weeks. Each baby develops at their own pace. Only about 5% of babies arrive on their due date, with most born within 2 weeks on either side.
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This calculator is for educational and informational purposes only and should not replace professional medical advice. Always consult with your healthcare provider for accurate pregnancy dating, prenatal care, and any concerns about your pregnancy. Gestational age calculations are estimates and individual pregnancies may vary.