Gestational Age Calculator

Gestational age is the standard way healthcare providers date pregnancy. It's measured from the first day of your last menstrual period, not from conception. This means you're considered "2 weeks pregnant" at conception, and pregnancy lasts 40 weeks (280 days) from your last period. Use our pregnancy week calculator to find out exactly how many weeks pregnant you are right now.

This dating system exists because most women know when their last period started, but don't know exactly when conception occurred. The two-week offset accounts for the time between your period and ovulation (which is when conception happens). If you want to calculate when you actually conceived, use our conception calculator to work backwards from your due date.

Medical professionals report gestational age in weeks and days format (like "12 weeks 3 days" or "12w3d"). This precision matters for timing prenatal tests, ultrasounds, and determining viability milestones. First trimester ultrasounds measuring Crown-Rump Length (CRL) can confirm or adjust your gestational age if your dates are uncertain. Once you know your gestational age, you can plan your full pregnancy journey with our pregnancy calendar.

The pregnancy dating system was established centuries ago, long before ovulation tracking existed. Most women could remember when their last period started but had no way to pinpoint ovulation or conception day. Doctors needed a consistent, universal starting point for pregnancy dating, so they chose the first day of the last menstrual period (LMP). This system stuck and is still used today even though we now have ways to track ovulation and conception more precisely.

The LMP dating system assumes a standard 28-day cycle with ovulation occurring on day 14. For women with regular 28-day cycles, this works perfectly - conception happens around day 14, making you "2 weeks pregnant" at conception, and your due date falls 280 days after your LMP. However, if you have shorter or longer cycles, ovulation shifts earlier or later than day 14, which means LMP-based dating can be off by several days or even weeks.

This is why first trimester ultrasounds are so valuable. Crown-Rump Length measurements between 8-13 weeks provide the most accurate pregnancy dating, regardless of your cycle length or when you actually conceived. If the ultrasound dating differs from your LMP dating by more than 7 days, your healthcare provider will adjust your due date based on the ultrasound. For women with irregular cycles or those who don't remember their LMP, ultrasound dating is essential. Calculate your due date from either LMP or conception date.

If you know your exact conception date (from tracking ovulation, timed intercourse, or fertility treatment), calculating gestational age is straightforward: add 2 weeks to the time since conception. For example, if conception occurred 6 weeks ago, your gestational age is 8 weeks. This conversion is necessary because all medical resources, prenatal tests, and fetal development information use gestational age, not conception age.

When you know your conception date, you can also calculate your "LMP equivalent" - a fictional date that makes your pregnancy align with standard dating. Your LMP equivalent is exactly 14 days before your conception date. For example, if you conceived on January 15, your LMP equivalent is January 1. This LMP equivalent is what goes in your medical records and what you'll use when talking to doctors or using pregnancy apps.

This becomes especially relevant for IVF pregnancies where conception date is precisely known. For IVF, your "conception date" is your egg retrieval date. The embryo transfer happens days later, but gestational age is calculated from retrieval, not transfer. A 5-day blastocyst transfer means you're 2 weeks 5 days pregnant on transfer day. Your IVF clinic will give you your LMP equivalent to use for standard pregnancy tracking.

First trimester ultrasounds use Crown-Rump Length (CRL) to determine gestational age. CRL measures the embryo from the top of the head (crown) to the bottom of the torso (rump), excluding the legs. Between 8-13 weeks, embryos of the same age are almost identical in size, making CRL an extremely reliable dating method. The ultrasound technician measures CRL in millimeters, and standardized tables convert that measurement to gestational age within days.

For example, a CRL of 45mm corresponds to 11 weeks 3 days gestation. A CRL of 60mm indicates 12 weeks 4 days. These measurements are so reliable that early ultrasound dating is considered the gold standard, more accurate than LMP-based dating. If you have irregular cycles, don't remember your LMP, or conceived during breastfeeding when cycles weren't regular, ultrasound CRL dating is the most trustworthy method available.

After 13 weeks, ultrasound dating becomes less accurate because babies start growing at individual rates. Head circumference, femur length, and abdominal circumference are used in second trimester, but these measurements have larger margins of error. This is why your due date should be set by your earliest ultrasound (ideally 8-13 weeks) and not changed based on later ultrasounds unless there's a significant discrepancy suggesting growth problems. Always use your first trimester dating for calculating gestational age. Track your pregnancy progress with our pregnancy month calculator.

Certain gestational ages mark important pregnancy milestones and medical decision points. At 6 weeks gestational age, a heartbeat should be visible on transvaginal ultrasound. At 8 weeks, the embryo officially becomes a fetus. At 12 weeks, you've completed the first trimester and miscarriage risk drops significantly. These early milestones are why precise gestational age dating matters - being even a week off can cause unnecessary worry if a heartbeat isn't seen "on time."

Viability begins around 24 weeks gestational age, the earliest point where a baby could potentially survive with intensive medical intervention if born. At 28 weeks, you enter the third trimester and babies born at this stage have much better outcomes. At 37 weeks, you're considered "term" and baby can be delivered safely if needed. At 39-41 weeks is "full term" - the ideal delivery window. These milestones guide medical decisions about monitoring, interventions, and delivery timing.

Prenatal testing windows are also based on gestational age. The nuchal translucency (NT) scan must be done between 11 weeks 0 days and 13 weeks 6 days - miss this window and the test can't be performed. The anatomy scan happens at 18-22 weeks. Glucose testing for gestational diabetes occurs at 24-28 weeks. Group B strep testing happens at 35-37 weeks. Knowing your precise gestational age ensures you don't miss these time-sensitive tests and screenings.

Healthcare providers will adjust your gestational age (and due date) if first trimester ultrasound dating differs significantly from LMP dating. The standard threshold is 7 days - if the ultrasound shows you're more than a week ahead or behind your LMP dates, they'll change your due date to match the ultrasound. This adjustment is common for women with irregular cycles, those who ovulated earlier or later than day 14, or those who weren't certain of their LMP date.

When your dates are adjusted, everything shifts - your current gestational age, your due date, and the timing of all prenatal tests. For example, if you thought you were 10 weeks based on LMP but ultrasound shows 9 weeks, you're really 9 weeks pregnant and your due date moves forward by one week. This adjustment might feel disappointing if you thought you were further along, but it ensures all your prenatal care is timed appropriately for your baby's actual development.

Once gestational age is set by a first trimester ultrasound, it rarely changes again. If later ultrasounds show baby measuring ahead or behind, that reflects individual growth patterns rather than incorrect dating. Some babies are bigger, some are smaller - this is normal variation. Unless measurements indicate a growth problem requiring intervention, your original gestational age and due date remain unchanged. Don't worry if your 20-week anatomy scan shows baby measuring 19 weeks or 21 weeks - a week's variation is normal and doesn't mean your dates are wrong.

Gestational age (counted from LMP) is always 2 weeks ahead of fetal age (counted from conception). When your doctor says you're 10 weeks pregnant, your baby is actually 8 weeks old since conception. This can be confusing, especially when you're researching fetal development online. Some resources describe development by gestational age, others by fetal age or "weeks since conception," so you need to know which system they're using.

For medical purposes, always use gestational age. When your doctor discusses viability, test timing, or developmental milestones, they're using gestational age. Pregnancy apps and books typically use gestational age too. However, if you're reading embryology research or detailed fetal development descriptions, they might use fetal age (weeks since conception). The 2-week difference matters - a "6-week embryo" in fetal age terms is actually 8 weeks gestational age.

This distinction becomes important when tracking early pregnancy after fertility treatment. If you had implantation 10 days ago and got a positive pregnancy test, you're about 3 weeks 3 days gestational age (counting from your LMP equivalent), but your embryo is only 10 days old since conception. Understanding this difference helps you interpret hCG levels, pregnancy symptoms, and ultrasound findings correctly. Use our hCG calculator to track early pregnancy hormone levels.

Twin and triplet pregnancies are dated the same way as singleton pregnancies - from LMP or by first trimester ultrasound CRL measurement. However, multiple pregnancies often deliver earlier than singletons. While a singleton pregnancy typically goes to 39-40 weeks, twins average 36-37 weeks delivery, and triplets often arrive by 33-34 weeks. This earlier delivery is normal and expected for multiples, not a sign of prematurity.

With twins, gestational age milestones shift slightly. Viability is similar to singletons (24 weeks), but "term" for twins is considered 37 weeks rather than 39 weeks. Many doctors recommend delivering twins by 38 weeks due to increased risks of complications after that point. For triplets, 35 weeks is often considered an appropriate delivery time if pregnancy progresses smoothly. These earlier "term" designations mean twins and triplets aren't truly premature even though they're born weeks before a singleton due date.

Monitoring is more intensive in multiple pregnancies. Starting in third trimester, you'll have more frequent ultrasounds to check growth of each baby and ensure they're developing proportionally. Gestational age becomes crucial for these assessments - doctors need to know if babies are growing appropriately for their gestational age or if growth discordance (one twin much smaller than the other) requires intervention. Accurate gestational age dating from early pregnancy ensures proper interpretation of all later measurements.

Once you know your gestational age, you can calculate all your important pregnancy dates. Your due date falls at 40 weeks gestational age. First trimester ends at 13 weeks. Second trimester spans 14-27 weeks. Third trimester runs from 28 weeks until delivery. Anatomy scan happens around 20 weeks. Viability milestone is 24 weeks. Full term begins at 37 weeks. These standardized timeframes help you anticipate what's coming and plan accordingly.

Gestational age also helps you track fetal development week by week. At 8 weeks gestational age, all major organs are formed. At 12 weeks, baby can move (though you can't feel it yet). At 20 weeks, you might start feeling movement. At 28 weeks, baby's lungs are developing rapidly. At 36 weeks, baby is essentially full-grown and just putting on weight. Knowing your exact gestational age lets you research what's happening with your baby right now and what to expect in coming weeks.

For practical planning, gestational age helps you time major life events around pregnancy. Want to travel? Second trimester (14-27 weeks) is safest. Planning maternity leave? Most women start around 36-38 weeks. Scheduling baby shower? 28-32 weeks is ideal - far enough along to feel secure but not so close to delivery that early labor could interrupt. Setting up nursery? Tackle that in third trimester when you're motivated by nesting instinct. Gestational age gives you the framework to plan your pregnancy timeline. Calculate when to expect each milestone with our pregnancy test calculator for early pregnancy or ovulation calculator for future cycles.