Gestational age counts from the first day of your last period. Fetal age, also called conception age, counts from conception itself, about two weeks later. So at "10 weeks pregnant," your baby's actual developmental age is closer to 8 weeks. Neither number is wrong, they're just counting from two different starting points, and that one gap explains most of the confusion people run into with scan reports, due date changes, and apps that seem to disagree with each other.
Gestational Age vs Fetal Age (and Conception Age), the 2 Week Gap
Doctors, pregnancy apps, and due date calculators almost always use gestational age by default. It's the clinical standard, counted from the first day of your last menstrual period (LMP), even though you weren't actually pregnant yet for the first two weeks of that count. Fetal age, or conception age, starts from when conception actually happened, which is typically around day 14 of an average cycle.
So if you ever see two different "weeks" numbers for what feels like the same pregnancy, one from a calculator using LMP and one from something counting since conception, they're both legitimate, just answering slightly different questions. If you want to work out your own conception date directly, our conception date calculator handles that math for you.
What "Weeks + Days" Means on a Scan Report (Like 8+4)
Scan reports and clinical notes often write gestational age as weeks plus days rather than a single number, for example 8+4. That means 8 completed weeks and 4 additional days, or 8 weeks and 4 days gestational age total. It's not a date or a measurement on its own, just a more precise way of writing the same gestational age number you'd see written as "8 weeks 4 days" elsewhere.
How Gestational Age Is Estimated: LMP vs Ultrasound (CRL)
There are two main ways gestational age gets estimated. The LMP based method counts forward from the first day of your last period, assuming a fairly typical cycle. The ultrasound based method measures your baby directly, most precisely in the first trimester using crown-rump length (CRL), the measurement from the top of the head to the bottom of the torso.
Ultrasound dating is generally considered more accurate than LMP based counting alone, especially if your cycles are irregular or you're not fully certain of your LMP date, since it's measuring your actual baby rather than estimating from an assumed average. You can run either method yourself with our gestational age calculator, which supports LMP, ultrasound measurement, conception date, due date, and corrected age for premature babies, five ways in one tool. For due date estimates specifically based on an ultrasound measurement, the ultrasound due date calculator is built for exactly that.
Why Your Dating Scan Changed Your Due Date
This is one of the most common sources of real confusion, and it's worth explaining clearly, since it catches a lot of people off guard. When an early ultrasound and an LMP based estimate disagree by a meaningful amount, providers often update the due date to match the scan rather than the original LMP based calculation. According to ACOG's guidance on estimating due dates, providers generally follow clinical standards that allow a larger acceptable difference between LMP and ultrasound dating later in pregnancy than earlier on, which is part of why a first trimester scan carries more weight for redating than a later one would.
If your due date shifted after a scan, that's usually the scan simply giving a more precise estimate than the original calendar based guess, not a sign that anything was done wrong the first time.
Measuring Ahead or Behind Later in Pregnancy, Size Isn't a New Due Date
Once your due date has been set, usually from your earliest accurate dating scan, later scans showing your baby measuring a bit ahead or behind typically reflect normal growth variation, not a reason to recalculate your due date. Babies naturally grow at somewhat different rates, and a single later measurement being a few days off from the "expected" size for that week is common and often not something to worry about on its own.
That said, size differences later in pregnancy can sometimes be worth a closer look depending on the specific numbers and your overall history, and that kind of interpretation genuinely needs your own provider, not a general explainer like this one. If a growth scan result is concerning you, the most useful next step is asking your provider directly what the specific numbers mean for your situation.
Which Number Doctors Actually Use (and Why Apps Confuse People)
Clinically, gestational age (LMP based, by convention) is the standard number used for due dates, appointment scheduling, and most pregnancy milestones you'll hear about. Part of why apps and trackers feel inconsistent is that some display gestational age while others occasionally show a conception based or fetal age number without labeling it clearly, so two apps can show different week counts for the same actual pregnancy, without either one being wrong. When in doubt, gestational age is the number your care team is almost certainly using.
Quick Check: Estimate Your Gestational Age
If you want to work this out for your own dates rather than just the general explanation, the gestational age calculator runs it instantly from whichever date you already know, your last period, a due date, an ultrasound measurement, a conception date, or corrected age if your baby was born premature. If you'd rather start from an estimated due date instead, the pregnancy due date calculator covers that, including a short explainer on how EDDs are calculated by method if you want the fuller breakdown. If you conceived via IVF, the IVF due date calculator accounts for your specific transfer date instead of LMP.
Frequently Asked Questions
Do doctors use gestational age or fetal age?
Gestational age. It's the clinical standard, counted from your last menstrual period, and it's the number used for due dates, appointment timing, and most milestones you'll hear referenced during your pregnancy.
Why do doctors "add two weeks" to my pregnancy?
They're not really adding anything, gestational age simply starts counting from your last period, about two weeks before conception actually happens. So your gestational age is naturally about two weeks ahead of your baby's actual developmental age from the start.
What is corrected age for premature babies?
Corrected age adjusts a premature baby's age by subtracting how many weeks early they were born, used mainly for understanding developmental milestones realistically. For specifics like vaccination schedules, providers typically use actual chronological age instead, so it's worth asking your pediatrician which age applies to which situation. For a full explanation and calculator, see our gestational age calculator's corrected age mode.
What does small for gestational age mean?
It describes a baby measuring smaller than typical for their gestational age at a given point in pregnancy. It's a specific clinical assessment that depends on multiple measurements and context, so if this comes up for you, it's best discussed directly with your provider rather than interpreted from a single number alone.
What's the difference between gestational age and fertilization age?
They refer to essentially the same concept, fertilization age (or fetal age) counts from conception, while gestational age counts from your last period, about two weeks earlier. You may see "fertilization age" used more in clinical or embryology contexts, while "fetal age" is the more common everyday phrasing.
Pregnancy dating methodology, including guidance on scan-based redating, per ACOG Committee Opinion #700, Methods for Estimating the Due Date.
Medical disclaimer. This article is for general educational purposes and is not a substitute for medical advice. Consult a healthcare provider for guidance specific to your situation.