How Many Weeks Pregnant Calculator
Enter the first day of your last menstrual period and your usual cycle length to estimate how many weeks pregnant you are, your due date, and the time remaining until that date.
Pregnancy Week, Due Date & Countdown Calculator
Pregnancy Week and Due Date Formula
Where:
- \(L\) — first day of the last menstrual period.
- \(C\) — cycle length in days.
- \(A\) — cycle-adjusted starting date.
- \(T\) — current date and time.
- \(N\) — completed days since the adjusted starting date.
- \(W\) and \(R\) — completed weeks and additional days of gestational age.
- \(E\) — estimated due date.
The calculator shifts the starting date by the difference between your cycle length and 28 days, then counts 280 days from that adjusted date to estimate a due date. It reports gestational age as completed weeks plus remaining days.
Pregnancy Week and Due Date – Calculation Example
Example: Suppose the first day of your last menstrual period was August 3, 2026, your cycle length is 28 days, and you calculate at the start of September 28, 2026.
\[ A=\text{August 3}+(28-28)\,\text{days}=\text{August 3, 2026} \]
\[ N=56\,\text{days}=8\times7+0,\qquad E=\text{August 3, 2026}+280\,\text{days}=\text{May 10, 2027} \]
Result: Gestational age is 8 weeks, 0 days, and the estimated due date is May 10, 2027. At the start of September 28, the due date is 32 weeks away. Later that day, the calculator may display 31 weeks, 6 days remaining because it counts full 24-hour periods.
How to use the calculator
- In First day of your last menstrual period, select the first day of bleeding from your most recent menstrual period—not the day it ended or a suspected conception date. The field initially shows today's date, so replace it with your actual date.
- In Cycle Length, enter your usual number of days from the first day of one period to the first day of the next. The default is 28 days; the accepted range is 20 to 45 days.
- Read Gestational age for completed weeks and days, Estimated Due Date for the projected delivery date, and Until Due Date for the time remaining.
What your pregnancy week means
Gestational age is the standard way to describe how far a pregnancy has progressed. It is counted from the first day of the last menstrual period, rather than from fertilization. In a typical 28-day cycle, ovulation occurs around two weeks after that date, so a gestational age of 8 weeks does not usually mean conception happened eight weeks ago.
The result is written as completed weeks and additional days. For example, 8 weeks, 3 days means eight full weeks have passed and you are three days into the next week. This dating helps place prenatal tests, scans, and fetal development in context. If you want another way to review pregnancy dating, see the Gestational Age Calculator.
A due date is an estimate, not an appointment for birth. This calculator starts with the conventional 40 weeks, or 280 days, and adjusts for cycle length: a 32-day cycle moves the estimated starting date and due date four days later than a 28-day cycle would. This assumes ovulation shifts with cycle length, which may not be true for every person. You can also explore due-date calculations with the Pregnancy Due Date Calculator.
Assumptions and practical limitations
Last-period dating is most useful when you know the date and your cycles are fairly regular. Bleeding that was not a menstrual period, an uncertain date, irregular cycles, recent hormonal contraception, or fertility treatment can make this estimate less suitable. Cycle length alone cannot identify the exact day of ovulation or conception. If you conceived through fertility treatment, your care team may use treatment dates instead.
An early ultrasound may provide a different gestational age or due date. A clinician can decide which date to use for prenatal care, particularly when the last-period date and scan disagree. For information based on ultrasound measurements, see the Ultrasound Gestation Period Calculator. Do not change a due date recorded by your care team solely because an online estimate differs.
The Until Due Date value is calculated from the current moment, while the due date is displayed as a calendar date. It can therefore look one day shorter than a simple count between calendar dates. If the estimated due date has passed, the remaining-time output may show negative weeks or days rather than an intuitive “overdue” label. A past due date does not, by itself, tell you when labor will begin; ask your maternity care team how your pregnancy should be monitored.
Frequently Asked Questions
How can I calculate how many weeks pregnant I am by hand?
Count the days from the first day of your last period to today, then subtract the number of days your cycle differs from 28 days. Divide the result by seven: the whole number is completed weeks, and the remainder is additional days. For example, if 66 days have passed and your cycle is usually 30 days, count 64 days, or 9 weeks and 1 day.
Is 40 weeks pregnant a normal time to give birth?
Forty weeks is an estimated due date, not a deadline. Birth from 37 weeks through 41 weeks and 6 days is generally described as term, with 39 weeks through 40 weeks and 6 days classified as full term. These ranges do not predict when an individual will give birth. Ask your maternity care team about monitoring or next steps if you approach or pass your due date.
How do I convert pregnancy weeks into months?
Pregnancy weeks do not convert neatly into months because calendar months vary in length. Dividing weeks by four gives only a rough approximation; for example, 20 weeks is closer to four and a half calendar months than five exact months. For appointments and prenatal tests, use weeks and days rather than months so there is no ambiguity.
What if I know when I tested positive but not when my last period started?
A positive test date alone cannot establish an accurate gestational age. Tests can become positive at different times after ovulation, and people test at different intervals. Avoid entering the test date as though it were the first day of a period. Tell your healthcare professional what dates you do know; an early ultrasound may help establish a more reliable pregnancy date.
Why might an early ultrasound give a different pregnancy week?
An early ultrasound estimates gestational age from measurements of the developing embryo, rather than assuming when ovulation occurred. The results can differ if ovulation happened earlier or later than expected or the period date is uncertain. Small differences are common; clinicians use dating guidelines to decide whether a due date should change. Keep the date your care team has recorded unless they advise otherwise.
How are pregnancy weeks counted after IVF embryo transfer?
IVF dating uses the embryo's age and transfer date, rather than an estimated last period. On the day a five-day embryo is transferred, gestational age is 2 weeks and 5 days; for a three-day embryo, it is 2 weeks and 3 days. Your fertility clinic can provide the due date used for your care. Do not substitute the transfer date for the last-period date in this calculator.
Which cycle length should I enter if it changes from month to month?
If your cycles are fairly consistent, use a typical length based on several recent cycles, counting from the first day of one period to the first day of the next. If lengths vary substantially, an average may not reflect when you ovulated in the cycle when pregnancy began. Treat the result as a rough estimate and discuss dating with your healthcare professional.