Am I really pregnant during weeks 1 and 2?

No. In weeks 1 and 2 you're having your period and then ovulating. Conception usually happens around the end of week 2, and implantation in week 3. The weeks are counted anyway so that every pregnancy is dated the same way.

What's the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last period and is what doctors, midwives and scans use. Fetal age (or conceptional age) is counted from conception and is about 2 weeks less. A baby at 12 weeks' gestation is about 10 weeks old from conception.

If I know my conception date, how many weeks pregnant am I?

Count the weeks since conception and add 2. For example, if you conceived exactly 6 weeks ago, you're about 8 weeks pregnant in medical terms. Or add 266 days to the conception date to get your due date, which gives the same result.

Does the 2-week difference apply to IVF?

Yes. IVF clinics also give dates in gestational weeks. On the day of a day-5 embryo transfer you're counted as 2 weeks and 5 days pregnant, because the embryo's age and the standard 2 weeks are both included.

Why Does Pregnancy Calculator Add 2 Weeks? (2026) / Momi

Q: Why does a pregnancy calculator add 2 weeks?

It doesn't really add them. Pregnancy is counted from the first day of your last period, and ovulation usually happens about 2 weeks later. So by the time you conceive, you're already counted as about 2 weeks pregnant. The calculator is simply using the standard medical starting point.

Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026

Quick answer: Why does a pregnancy calculator add 2 weeks? Because pregnancy is counted from the first day of your last period, not from conception. Ovulation usually happens about 2 weeks after your period starts, so on the day you conceive you're already counted as roughly 2 weeks pregnant. Doctors call this gestational age.

So if you know you conceived four weeks ago and an app says you're six weeks pregnant, nothing is wrong. Here's why the 2 weeks are there, what they mean for your due date and scans, and how to convert between the two ways of counting.

Gestational age vs fetal age

There are two ways to count how far along a pregnancy is:

	Gestational age	Fetal (conceptional) age
Counted from	First day of your last period (LMP)	Conception (fertilisation)
Used by	Doctors, midwives, scans, due date calculators, apps	Embryology textbooks, some IVF descriptions
Full term	40 weeks	38 weeks
Example: 12-week scan	12 weeks	About 10 weeks since conception

Every number you'll hear in pregnancy care is gestational age: the 12-week scan, the 20-week scan, "full term at 39 weeks", even the weeks in a pregnancy week by week guide. The American College of Obstetricians and Gynecologists (ACOG) dates pregnancy from the LMP, then confirms it with an early ultrasound.

Where the 2 weeks come from

Here's a typical 28-day cycle, counted the medical way:

Week 1: your period. You're not pregnant, but this is "week 1" of pregnancy.
Week 2: the egg matures and is released at ovulation, around day 14.
End of week 2: the egg is fertilised if sperm are present.
Week 3: the fertilised egg travels to the uterus and implants.
Week 4: your period is due. Pregnancy hormone (hCG) has risen enough for many home tests to turn positive. The NHS says tests are most reliable from the first day of a missed period.

So the "extra" 2 weeks are the time between your period starting and ovulation. They're counted because the start of a period is a date most people know. The exact day you ovulated or conceived usually isn't.

Why not just count from conception?

It sounds more logical, but in practice it's harder:

Most people don't know the day they conceived. Sperm can survive for several days, so sex on one day can lead to conception days later.
Ovulation varies. Even with a 28-day cycle, ovulation can move around from month to month.
A shared starting point. Counting from the LMP means a midwife, a sonographer and a hospital in another city all use the same week numbers.
The research is built on it. Growth charts, screening windows and term dates were all defined in gestational weeks.

Once you have a first-trimester scan, the measurement of your baby replaces the guesswork. The scan date is then given in gestational weeks too, keeping the 2-week offset.

What this means for week-by-week apps and calculators

Because every app counts from your LMP:

Week 4 = about 2 weeks after conception. This is when many people get a positive test.
A "6-week" scan is about 4 weeks after conception.
Your due date is 40 weeks from your LMP, or 38 weeks (266 days) from conception. The due date calculator handles both.
Your baby's size each week is based on gestational age, matching scan reports.

Converting between the two

You know	To get gestational age	Example
Weeks since conception	Add 2 weeks	Conceived 6 weeks ago → about 8 weeks pregnant
Conception date	Due date = conception + 266 days	Conceived 1 March → due about 22 November
Ovulation date (tracked)	Treat as conception, add 266 days	See the due date calculator from conception
IVF transfer date	Transfer + 266 − embryo age	Day-5 transfer → transfer + 261 days

If your cycles are longer than 28 days, you probably ovulate later than day 14. Then your "real" pregnancy is a little shorter than the LMP count suggests, and your scan may move your due date later. Our guide to how accurate pregnancy calculators are covers irregular cycles.

Does it change anything medically?

No. The 2-week offset is a counting convention. It doesn't mean your pregnancy is longer, and it doesn't affect your baby. What matters is that you and your care team use the same date. After your dating scan, use the due date your midwife or doctor gives you. See can your due date change? for when that happens.

It's also why early pregnancy facts can sound confusing. "At 5 weeks, the heart starts to form" means 5 weeks from your LMP, about 3 weeks after conception. Mayo Clinic's first-trimester development guide counts the same way.

Quick worked example

Say your last period started on 2 March 2026 and your cycle is 28 days.

You likely ovulated and conceived around 16 March.
On 16 March you're 2 weeks pregnant (gestational age).
On 30 March, when your period is late, you're 4 weeks pregnant.
Your due date is 2 March + 280 days = 7 December 2026. From conception, 16 March + 266 days gives the same date.

To find your exact week and day today, use How Many Weeks Pregnant Am I?. For the full method, read how is due date calculated.

How Momi counts your weeks

Momi uses the standard medical count. Enter the first day of your last period, or a known conception date, and Momi works out your due date and current week, then follows your pregnancy week by week with your baby's size and development. If your dating scan gives a new date, update it so your timeline matches your notes.

Download Momi free on the App Store or Google Play. Free with ads; optional Premium.

Sources

ACOG, Committee Opinion 700: Methods for Estimating the Due Date
NHS, Pregnancy due date calculator
NHS, Doing a pregnancy test
Mayo Clinic, Fetal development: The first trimester

Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.

Why Does a Pregnancy Calculator Add 2 Weeks?