Current Gestational Age
XX weeks X days
Weeks
Days
Weeks Left
Trimester
Important Dates
Gestational Age vs Fetal Age
| Gestational Age | Fetal Age | Milestone |
|---|---|---|
| 4 weeks | 2 weeks | Implantation |
| 6 weeks | 4 weeks | Heartbeat visible |
| 8 weeks | 6 weeks | Embryo → fetus |
| 12 weeks | 10 weeks | First trimester ends |
| 20 weeks | 18 weeks | Anatomy scan |
| 24 weeks | 22 weeks | Viability |
| 40 weeks | 38 weeks | Due date |
Gestational age is always 2 weeks ahead of fetal age (age from conception).
How Gestational Age Is Calculated: The Complete Guide
When you find out you're pregnant, one of the first questions is "how far along am I?" The answer involves gestational age—a measurement that confuses nearly every first-time parent. Your doctor tells you you're 6 weeks pregnant, but you know you conceived just 4 weeks ago. The math doesn't add up until you understand how pregnancy dating actually works. Gestational age counts from the first day of your last menstrual period (LMP), not from conception. This means you're technically "2 weeks pregnant" at the moment of conception, and those first two weeks of pregnancy happened before your baby even existed.
Gestational Age vs. Fetal Age: The Two-Week Gap
The confusion between gestational age and fetal age trips up countless expectant parents. Gestational age (what doctors use) counts from your LMP, while fetal age (also called embryonic age) counts from actual conception. The difference is always about two weeks. If your provider says you're 12 weeks pregnant, your baby has been developing for roughly 10 weeks. This dating convention exists because pinpointing conception is surprisingly difficult—sperm can survive inside the body for up to five days, and ovulation timing varies. Your LMP, however, is a date most people can identify with confidence. Medical professionals worldwide use gestational age so everyone speaks the same language when discussing pregnancy milestones, test timing, and fetal development.
Why Your Ultrasound Date Might Differ From Your LMP
You calculated your due date from your last period, but then your dating ultrasound shows you're measuring a week behind—or ahead. This happens more often than you'd expect. The LMP calculation assumes a textbook 28-day cycle with ovulation on day 14, but only about 16% of women actually have that cycle length. If you have a 35-day cycle, you likely ovulated around day 21, making your pregnancy about a week younger than LMP would suggest. Late ovulation, early ovulation, irregular cycles, and even implantation timing can all shift your actual gestational age. First-trimester ultrasounds measure your baby's crown-rump length (CRL) and can estimate gestational age within 5-7 days—far more accurate than LMP calculations for most women. According to clinical guidelines, if the ultrasound differs from LMP dating by more than 7 days before 14 weeks, your provider will adjust your due date to match the ultrasound.
Understanding the Weeks and Days Notation
Medical records and pregnancy apps use a precise notation you'll see throughout your care: "12+3" or "12 3/7" both mean 12 weeks and 3 days pregnant. The days count from 0 to 6 within each week. So 12+0 means exactly 12 completed weeks, 12+3 means 12 weeks plus 3 days, and 12+6 is the last day before reaching week 13. This precision matters because critical developments and test windows happen within narrow timeframes. The nuchal translucency screening, for example, must occur between 11+0 and 13+6 weeks. Viability—when survival outside the womb becomes possible—is typically defined at 24+0 weeks. One day can change whether certain interventions are offered. When tracking your pregnancy week by week, our pregnancy week calculator shows you exactly where you stand in this notation.
How Doctors Measure Your Baby's Size for Dating
During your first-trimester ultrasound, the sonographer measures your baby from crown (top of head) to rump (bottom)—this CRL measurement is remarkably consistent across all healthy pregnancies in early development. A 23mm CRL corresponds to about 9 weeks gestational age, regardless of parental height, ethnicity, or other factors. This standardization makes early ultrasounds incredibly reliable for dating. After 14 weeks, babies start growing at individual rates influenced by genetics, and CRL becomes less accurate. Second-trimester dating uses head circumference (HC) and biparietal diameter (BPD) instead, but these measurements carry wider margins of error—plus or minus 10-14 days. By the third trimester, ultrasound dating can be off by 2-3 weeks because fetal growth variation is too significant. This is why your provider likely won't change your due date based on later ultrasounds, even if baby is measuring "big" or "small."
When and Why Your Due Date Gets Adjusted
Many expecting parents experience the disconcerting moment when their provider changes their due date after an ultrasound. The guidelines are specific: before 9 weeks, a difference of more than 5 days warrants redating; between 9-14 weeks, more than 7 days triggers a change; and in the second trimester, the threshold increases to more than 14 days. Once established in the first trimester, your due date typically stays fixed—even if later ultrasounds show baby measuring ahead or behind. Measuring large or small later in pregnancy reflects growth patterns, not dating errors. Your official estimated due date (EDD) anchors all subsequent care decisions, from genetic screening windows to determining post-term status. If your LMP was uncertain or your cycles irregular, an early dating ultrasound becomes especially valuable. Need to know when you conceived? Our conception calculator can help you work backward from your due date.
Dating Accuracy Changes Throughout Pregnancy
The accuracy of gestational age dating depends heavily on when it's established. First-trimester CRL measurements (before 14 weeks) are accurate within plus or minus 5-7 days—this is as precise as pregnancy dating gets. Between 14-20 weeks, accuracy drops to plus or minus 10-14 days. From 20-30 weeks, expect a margin of plus or minus 2-3 weeks. Beyond 30 weeks, dating accuracy stretches to plus or minus 3-4 weeks because fetal size variation becomes substantial. This declining precision is why early prenatal care matters so much. Women who have their first ultrasound in the third trimester may have significantly uncertain dates, which complicates decisions about interventions near term. If you're planning a pregnancy or just found out you're expecting, scheduling that first appointment early—ideally between 7-12 weeks—gives you the most reliable dating possible. For calculating your expected delivery date, our due date calculator uses these same dating principles.
Frequently Asked Questions
Why am I considered 4 weeks pregnant when I just found out?
When you miss your period and get a positive test, you're typically around 4 weeks pregnant by gestational age—even though conception happened only about 2 weeks ago. Pregnancy dating starts from your last period, not from conception. Those first '2 weeks of pregnancy' are actually just your regular menstrual cycle leading up to ovulation. This confuses almost everyone at first, but it's the universal standard that allows consistent communication between healthcare providers worldwide.
My ultrasound says I'm measuring a week behind—should I be worried?
A one-week difference between LMP dating and ultrasound is extremely common and usually not concerning. It typically means you ovulated later than the assumed day 14 of your cycle—perhaps you have slightly longer cycles, or ovulation was delayed that particular month. If the difference is less than 7 days, your provider may keep your original due date. Differences greater than 7 days in the first trimester usually result in adjusting your dates to match the ultrasound, which is more accurate than LMP calculations.
What does CRL mean on my ultrasound report?
CRL stands for Crown-Rump Length—the measurement from the top of your baby's head to the bottom of the buttocks. In early pregnancy (before 14 weeks), this measurement is the gold standard for determining gestational age because all embryos and fetuses grow at remarkably similar rates during this period. A formula converts your baby's CRL in millimeters to gestational age in weeks and days. After 14 weeks, CRL becomes less reliable, and other measurements like head circumference take over.
Why won't my doctor change my due date even though baby is measuring big?
Once your due date is established in the first trimester, providers typically don't change it based on later ultrasounds. Measuring 'big' or 'small' in the second and third trimesters reflects your baby's individual growth pattern—influenced by genetics, nutrition, and other factors—not dating errors. Your 36-week baby measuring at 38 weeks' size doesn't mean you'll deliver early; it means you may have a larger-than-average baby. Changing due dates based on later measurements could actually create problems with managing your care.
Is gestational age calculated differently for IVF pregnancies?
IVF pregnancies use the same gestational age framework, but the calculation is more precise because the fertilization date is known exactly. For a Day 5 blastocyst transfer, you're considered 2 weeks and 5 days pregnant on transfer day—the 2 weeks accounts for the standard LMP-to-ovulation period, plus the 5 days the embryo developed in the lab. Day 3 transfers add 2 weeks and 3 days. This precision makes IVF dating extremely accurate, and ultrasound dating usually matches perfectly.
How accurate is my due date, really?
Only about 4-5% of babies arrive on their exact due date. The due date is the midpoint of a normal delivery window, not a precise prediction. Full-term pregnancy spans from 37 to 42 weeks, meaning your baby could arrive anywhere in that 5-week range and still be considered on time. First babies tend to arrive a few days later than the due date on average, while subsequent pregnancies often deliver slightly earlier. Think of your due date as a useful reference point rather than an appointment your baby has agreed to keep.