Pregnancy lasts about 266 days (38 weeks) from the date of conception. That’s roughly two weeks shorter than the 40-week figure you’ll hear at most doctor’s appointments, because the medical system counts from the first day of your last menstrual period, not from when you actually conceived. Since conception typically happens about two weeks after that period starts, the difference is purely a matter of where the clock begins.
Why the 38-Week and 40-Week Numbers Differ
Doctors use the “last menstrual period” (LMP) method because most people know when their period started but don’t know exactly when they ovulated or conceived. Counting from the LMP adds roughly 14 days to the front of the pregnancy, which is why the standard medical timeline is 40 weeks even though a baby actually develops for closer to 38.
If you know your conception date, whether from ovulation tracking, an ovulation predictor kit, or IVF, you can estimate your due date by counting 266 days (38 weeks) forward from that date. If your pregnancy resulted from IVF or another form of assisted reproduction, your provider will typically use the embryo transfer date and embryo age to assign a due date rather than relying on the LMP formula.
What the Research Says About Average Length
A well-known study published through the European Society of Human Reproduction and Embryology tracked pregnancies where the exact date of ovulation was confirmed. The median time from ovulation to birth was 268 days, or 38 weeks and 2 days. Conception happens within 12 to 24 hours of ovulation, so the difference between measuring from ovulation and measuring from conception is negligible.
The more striking finding was the variation. Even after excluding preterm births and pregnancies with complications, the range in pregnancy length spanned 37 days. That means two perfectly healthy, uncomplicated pregnancies could differ by more than five weeks. This is why a “due date” is really a due window. Only about 4 to 5 percent of babies arrive on their predicted date.
What Affects How Long Your Pregnancy Lasts
Several factors push pregnancy slightly longer or shorter. Maternal genetics and fetal genetics both play a role, meaning your family history of pregnancy length is somewhat predictive of your own. Smoking during pregnancy is associated with shorter gestation. The baby’s sex can also influence timing slightly. First pregnancies tend to run a bit longer than subsequent ones.
Older maternal age has been linked to longer pregnancies in some studies, though the effect is modest. None of these factors change the due date by more than a few days on average, but they help explain why your experience may not match someone else’s.
Trimesters Counted From Conception
The standard trimester breakdown is built around the 40-week LMP calendar, so the weeks shift if you’re counting from conception instead.
- First trimester: Conception through about 10 weeks post-conception (weeks 1 through 12 on the LMP calendar).
- Second trimester: About 11 to 25 weeks post-conception (weeks 13 through 27 LMP).
- Third trimester: About 26 to 38 weeks post-conception (weeks 28 through 40 LMP).
Most pregnancy apps, ultrasound reports, and medical records use the LMP-based week count. If your provider says you’re “12 weeks pregnant,” you conceived roughly 10 weeks ago. Keeping this two-week offset in mind helps avoid confusion when you compare your conception date to the gestational age on your chart.
The First Days After Conception
Fertilization itself happens quickly. Once an egg is released during ovulation, it survives 12 to 24 hours. If sperm reaches it in that window, conception occurs. The fertilized egg then spends about six days traveling down the fallopian tube before implanting into the uterine lining. Implantation is the event that triggers your body to start producing hCG, the hormone that pregnancy tests detect.
Because hCG needs time to build up in your blood and urine, most home pregnancy tests are reliable starting on the first day of your missed period, which is roughly 14 days after conception. Some sensitive tests can pick up hCG a few days earlier. If you’re unsure when your period is due, testing at least 21 days after unprotected sex gives the most dependable result.
How to Calculate Your Due Date From Conception
If you’re confident in your conception date, the math is straightforward: add 266 days (or 38 weeks) to that date. Many online calculators let you enter a conception date directly and will do the arithmetic for you. Keep in mind that if your first ultrasound suggests a different gestational age by more than a few days, your provider may adjust the due date based on the baby’s measurements, since early ultrasounds are highly accurate for dating.
Regardless of how the due date is calculated, full-term pregnancy is defined as delivery between 39 weeks 0 days and 40 weeks 6 days on the LMP scale, which corresponds to roughly 37 to 39 weeks after conception. Babies born between 37 and 39 LMP weeks (35 to 37 weeks post-conception) are considered “early term,” and those born after 41 LMP weeks are “late term.” All of these outcomes fall within the normal range of human pregnancy.