Pregnancy & Time
Pregnancy Age Calculator: Gestational Weeks, Due Dates & the Math Behind It All
Pregnancy is measured in weeks, but the starting line isn't when you think it is. Here's exactly how gestational age is calculated, why doctors count backward from your last period, and how to find your due date.
If you've ever wondered "how many weeks pregnant am I?" the answer depends on a specific calculation that surprises most people: doctors don't count from conception. They count from the first day of your last menstrual period (LMP), which means you're technically "pregnant" for about two weeks before conception actually occurs.
Why Count from the Last Period?
The LMP method exists because conception is difficult to pinpoint precisely — ovulation typically occurs 12–16 days after the start of a period, and fertilization can happen up to 5 days after intercourse. The LMP date is known and consistent.
This means a full-term pregnancy is 40 weeks from LMP, but only about 38 weeks of actual fetal development from conception. Both numbers are correct — they just measure different things.
How to Calculate Gestational Age
The formula:
Weeks pregnant = (Today's date − First day of LMP) ÷ 7
Example: LMP = Jan 1, 2026 · Today = Mar 15, 2026
Mar 15 − Jan 1 = 73 days ÷ 7 = 10 weeks, 3 days pregnant
The result is your gestational age expressed as "X weeks and Y days." Obstetricians use this number to track fetal development milestones, schedule screenings, and calculate when your due date falls.
Due Date Calculation: Naegele's Rule
The standard due date formula is called Naegele's Rule, named after German obstetrician Franz Karl Naegele (1778–1851). It's elegantly simple:
Naegele's Rule:
Due Date = LMP + 280 days (40 weeks)
Or equivalently:
Due Date = LMP + 9 months + 7 days
Assumes a 28-day cycle with ovulation on day 14. Adjust for longer/shorter cycles.
Pregnancy Trimesters: The Time Breakdown
First Trimester
Weeks 1–12- ·Weeks 1–2: Pre-conception (counting from LMP)
- ·Week 4: Implantation; pregnancy test positive
- ·Week 6: Heartbeat detectable via ultrasound
- ·Week 8: Embryo becomes a fetus
- ·Week 12: Risk of miscarriage drops significantly
Second Trimester
Weeks 13–26- ·Week 14: Gender may be visible on ultrasound
- ·Week 18–20: Anatomy scan (detailed ultrasound)
- ·Week 20: Typically when most people 'announce'
- ·Week 24: Viability threshold — fetus can potentially survive outside womb
- ·Week 26: Surfactant production begins in lungs
Third Trimester
Weeks 27–40- ·Week 28: Third trimester begins; weekly monitoring increases
- ·Week 32: Most babies are head-down by now
- ·Week 37: 'Early term' — lungs nearly mature
- ·Week 39–40: 'Full term' — optimal development window
- ·Week 42: Post-term; induction typically recommended
How Accurate Is the Due Date?
Less accurate than most people expect. Only about 4–5% of babies are born on their exact due date. The majority arrive within two weeks before or after. Here's how the distribution actually breaks down:
First-time mothers tend to deliver later than average; subsequent pregnancies tend to be slightly shorter. Twins and multiples arrive earlier on average (around 36–37 weeks). Ultrasound measurements, particularly those taken in the first trimester, are the most accurate for confirming gestational age.
Adjusted Age for Premature Babies
For babies born before 37 weeks, pediatricians use a corrected age (also called adjusted age) when tracking development milestones. The formula:
Corrected Age = Actual Age − (40 weeks − gestational age at birth)
Example: Baby born at 32 weeks, now 6 months old
Corrected Age = 6 months − 8 weeks (2 months) = ~4 months
This corrected age is used until roughly 2 years of age, after which the developmental gap typically closes and the adjustment is no longer clinically significant.
Calculate age from any date
Whether you're tracking a pregnancy, a baby's age in weeks, or anything else — the Age Since calculator lets you enter any start date and see the elapsed time in years, months, weeks, days, and more.
Open Age Calculator →