Calculate your pregnancy due date, gestational age, current pregnancy week, trimester, estimated conception date, IVF due date, and pregnancy timeline from the information you know.
Pregnancy Calculator
What do you know? Choose a method below, then press Calculate.
days
The standard 280-day calculation assumes a 28-day menstrual cycle. If your average cycle length is different, this calculator adjusts the estimate by the difference from 28 days. A 30-day cycle shifts the estimate about 2 days later; a 26-day cycle shifts it about 2 days earlier. This adjustment does not make the estimate medically exact.
Enter the due date your healthcare provider has already given you. This calculator will work backward to estimate your current gestational age, trimester, approximate conception date, and pregnancy timeline. These backward-calculated dates are estimates.
Obstetric gestational age will still read about two weeks greater than conception age, because standard pregnancy dating counts from the menstrual-cycle convention rather than from fertilization.
The embryo transfer date is not treated as the date of conception. ACOG guidance notes that when a pregnancy results from assisted reproductive technology, the ART-derived gestational age should be used for the estimated due date.
Gestational Age Shown on Ultrasound
weeks
days
Enter the gestational age your ultrasound report or healthcare provider gave you.
This calculator does not interpret ultrasound images or measurements such as crown-rump length. Early ultrasound is commonly used to establish or confirm pregnancy dating, but your healthcare provider's official estimated due date should always be followed over this or any online calculator.
Your Pregnancy Timeline
This calculator provides educational estimates and does not replace prenatal care or the due date established by your healthcare professional.
Pregnancy Due Date Calculator: How to Calculate Your Due Date and Pregnancy Week
A pregnancy due date calculator gives you an estimated date for when your baby may arrive, but the date itself is only one part of the story. A useful pregnancy calculator should also help answer the questions that usually come next: How many weeks pregnant am I? What trimester am I in? When might conception have occurred? How is a due date calculated from my last period? Does IVF use the same calculation? And why might an ultrasound give a different date?
Pregnancy dating can feel confusing because pregnancy is traditionally counted from the first day of the last menstrual period, not from the actual day conception occurred. That means the pregnancy clock usually begins roughly two weeks before conception in a typical 28-day cycle. This convention allows healthcare professionals to use a consistent dating system even when the exact date of fertilization is unknown.
This CalculateThisWay Pregnancy Calculator brings the most common pregnancy dating methods into one place. You can calculate from your last menstrual period, a known conception date, an IVF embryo transfer, an existing due date, or the gestational age recorded during an ultrasound.
Visual: Your Pregnancy Dating Timeline
Pregnancy is conventionally dated from the first day of the last menstrual period (LMP), roughly two weeks before ovulation and conception, continuing through three trimesters to an estimated due date around 40 weeks.
Pregnancy dating timeline from LMP to estimated due date
Point on Timeline
Approximate Gestational Age
First day of last menstrual period
0 weeks 0 days
Approximate ovulation and conception
About 2 weeks
End of first trimester
13 weeks 6 days
End of second trimester
27 weeks 6 days
Estimated due date
40 weeks 0 days
How Is a Pregnancy Due Date Calculated?
The most common pregnancy due date calculation begins with the first day of the last menstrual period, commonly shortened to LMP. A standard pregnancy is dated as approximately 280 days, or 40 weeks, from that date.
For a person with a 28-day cycle:
Estimated Due Date = First Day of LMP + 280 Days
Another traditional way to describe the same method is Naegele's rule: add seven days to the first day of the last menstrual period, subtract three months, and add one year when necessary. Modern calculators simply perform the date arithmetic automatically.
The 280-day method assumes a menstrual cycle around 28 days and ovulation around the middle of that cycle. Real cycles vary, which is why this calculator allows an average cycle-length adjustment. If your normal cycle is 30 days, for example, the estimate can be shifted approximately two days later. If your cycle averages 26 days, it may shift approximately two days earlier.
This does not turn an LMP estimate into an exact prediction. Ovulation can occur earlier or later than expected, cycles may change from month to month, and the date remembered as the last menstrual period may not always represent a typical cycle.
Visual: 28-Day Cycle vs. 30-Day Cycle
A longer average cycle shifts the estimated due date later; a shorter average cycle shifts it earlier. A 30-day cycle moves the estimate about 2 days later than the standard 28-day calculation.
How average cycle length shifts the estimated due date
Average Cycle Length
Adjustment to Standard 280-Day Estimate
26 days
About 2 days earlier
28 days (standard)
No adjustment
30 days
About 2 days later
Why Pregnancy Is Counted Before Conception
One of the most common pregnancy questions is, "How can I be four weeks pregnant if conception happened only about two weeks ago?"
The answer is the way gestational age is defined.
Gestational age is conventionally counted from the first day of the last menstrual period. Conception age, sometimes called fetal age in casual explanations, begins around fertilization. Because ovulation often occurs approximately two weeks after the start of a typical 28-day cycle, gestational age is usually around two weeks ahead of conception age.
For example, if conception occurred approximately six weeks ago, the pregnancy may be described clinically as about eight weeks gestational age.
The exact gap will not be two weeks for everyone because cycle length and ovulation timing vary.
Visual: Gestational Age vs. Approximate Conception Age
Gestational age starts from the first day of the last menstrual period. Approximate conception (fetal) age starts about two weeks later, around ovulation and fertilization. This is why someone described as 8 weeks pregnant is generally about 6 weeks from conception.
Gestational age compared with approximate conception age
Dating System
Starts Counting From
Gestational age
First day of the last menstrual period
Conception / fetal age
Approximately 2 weeks later, around fertilization
How to Calculate a Pregnancy Due Date From Conception
If the conception date is known with reasonable confidence, the estimated due date can be calculated by adding approximately 266 days, or 38 weeks.
Estimated Due Date = Conception Date + 266 Days
This calculation is shorter than the 280-day LMP method because it does not include the approximately two weeks before conception that are built into standard gestational dating.
A known conception date may be useful when ovulation was carefully tracked, but outside assisted reproduction, the exact moment of fertilization is often difficult to identify. Intercourse date is also not necessarily conception date because sperm can remain capable of fertilization for several days.
If you are still trying to estimate the likely fertile period rather than dating an established pregnancy, use the CalculateThisWay Ovulation Calculator.
Visual: LMP + 280 Days vs. Conception + 266 Days
Both methods are built to land on the same estimated due date for a typical cycle: 280 days from LMP equals 266 days from conception, because conception is conventionally about 14 days after LMP.
LMP-based dating compared with conception-based dating
Starting Point
Days Added to Reach the Estimated Due Date
First day of last menstrual period
280 days
Conception date
266 days
IVF Due Date Calculator: Day 3 and Day 5 Embryo Transfers
Pregnancies created through in vitro fertilization are dated differently because the embryo transfer date and embryo age are known.
For a Day 3 embryo transfer:
Estimated Due Date = Transfer Date + 263 Days
For a Day 5 blastocyst transfer:
Estimated Due Date = Transfer Date + 261 Days
Both formulas come from the same underlying principle. A pregnancy is approximately 266 days from fertilization to the estimated due date. Because the embryo is already three or five days old at transfer, those days are subtracted from 266.
This also means an IVF pregnancy does not begin at "zero weeks" on transfer day. On a Day 3 transfer, standard gestational dating is approximately 2 weeks 3 days. On a Day 5 blastocyst transfer, it is approximately 2 weeks 5 days.
Assisted-reproduction dating should ultimately follow the date established by the fertility clinic and obstetric provider.
Visual: Day 3 Transfer (+263 Days) vs. Day 5 Transfer (+261 Days)
From the same transfer date, a Day 5 blastocyst transfer reaches its estimated due date two days sooner than a Day 3 transfer, because the embryo was already two days further along when it was transferred.
IVF due date offsets by embryo age at transfer
Embryo Age at Transfer
Days Added to Transfer Date
Gestational Age on Transfer Day
Day 3 embryo
263 days
2 weeks 3 days
Day 5 blastocyst
261 days
2 weeks 5 days
How Far Along Am I in Pregnancy?
When someone asks "How far along am I?", the answer is usually expressed as gestational weeks and days.
A result such as:
12 weeks 4 days
means 12 complete weeks and four additional days of gestational age.
A person who is 12 weeks 4 days pregnant is currently in the 13th week of pregnancy, even though only 12 full weeks have been completed.
That distinction is why this calculator should display both:
Gestational Age: 12 weeks 4 days
and
Current Pregnancy Week: Week 13
This makes the answer easier to understand when comparing calculator results with pregnancy apps or prenatal appointment information.
Visual: Completed Weeks vs. Current Pregnancy Week
12 weeks 4 days means 12 complete weeks have passed, with 4 additional days counted into the 13th week, so the current pregnancy week is Week 13.
Example: completed gestational weeks compared with current pregnancy week
Measurement
Example Value
Gestational age (completed weeks and days)
12 weeks 4 days
Current pregnancy week
Week 13
What Are the Three Trimesters of Pregnancy?
Pregnancy is commonly divided into three trimesters.
The first trimester runs from the beginning of gestational dating through 13 weeks and 6 days.
The second trimester begins at 14 weeks 0 days and continues through 27 weeks 6 days.
The third trimester begins at 28 weeks 0 days and continues through the remainder of the pregnancy.
These divisions are useful because prenatal care, screening windows, fetal development, and common pregnancy experiences change throughout pregnancy.
Rather than making someone calculate these dates manually, this Pregnancy Calculator should automatically place the estimated pregnancy into the correct trimester and display the approximate calendar dates for all three. Calculate your dates above to see your own personalized trimester cards and milestone timeline.
Visual: The Three Trimesters
First trimester: 0 weeks 0 days through 13 weeks 6 days. Second trimester: 14 weeks 0 days through 27 weeks 6 days. Third trimester: 28 weeks 0 days through the estimated due date around 40 weeks.
Standard trimester boundaries used by this calculator
Trimester
Gestational Age Range
First trimester
0 weeks 0 days through 13 weeks 6 days
Second trimester
14 weeks 0 days through 27 weeks 6 days
Third trimester
28 weeks 0 days onward
What Does Estimated Due Date Mean?
An estimated due date, often abbreviated EDD, is not a guarantee that labor will begin or that a baby will be born on that exact day.
It is better to think of the EDD as the center point of a pregnancy dating system.
The Better Health Channel's baby due date guide explains that pregnancy is traditionally calculated as about 40 weeks from the first day of the last menstrual period and notes that only a small percentage of births occur on the exact estimated date.
Because a due date is an estimate, the calculator result should be used as a planning and educational tool rather than as a prediction of the exact birth date.
Visual: Your Estimated Delivery Window
A due date is a reference point, not an appointment.
The estimated due date sits at the center of a broader delivery window, not a fixed deadline
Position
Description
Earlier than estimated due date
Many births occur in this part of the window
Estimated due date
Center reference point of the dating system
Later than estimated due date
Many births also occur in this part of the window
Can an Ultrasound Change a Pregnancy Due Date?
An early ultrasound can help establish or confirm gestational age, especially when the last menstrual period is uncertain, menstrual cycles are irregular, or LMP dating does not align with ultrasound findings.
Early pregnancy ultrasound uses fetal or embryonic measurements to estimate gestational age. A clinician then considers the ultrasound findings together with menstrual history and other pregnancy information.
This calculator should not attempt to interpret ultrasound images or fetal measurements.
Instead, the Ultrasound Dating mode allows a user to enter the ultrasound date and the gestational age already provided on the ultrasound report or by the healthcare professional.
The calculator can then translate that documented gestational age into an estimated due date and pregnancy timeline.
The healthcare provider's official dating should take priority over an online calculator.
Visual: Ultrasound Dating Flow
The calculator takes the ultrasound date together with the documented gestational age from that same visit, then works out the estimated due date and remaining weeks to 40 weeks.
How ultrasound dating mode calculates an estimated due date
Step
What Happens
1. Ultrasound date entered
The calendar date of the scan
2. Documented gestational age entered
Weeks and days as reported on that date
3. Calculation
Remaining days to 40 weeks are added to the ultrasound date
4. Result
Estimated due date and full pregnancy timeline
Pregnancy Due Date Example Using the Last Period
Worked Example
Suppose the first day of the last menstrual period was January 1 and the average cycle length is 28 days.
Step 1: Start with January 1.
Step 2: Add 280 days.
Step 3: The resulting calendar date is the estimated due date.
If the person's average cycle is 30 days instead of 28, the calculator adds approximately two additional days.
This example illustrates why cycle length can matter, but it also shows why a due date should remain an estimate. The method depends on assumptions about ovulation timing.
Pregnancy Due Date Example From Conception
Worked Example
Suppose conception is estimated to have occurred on February 1.
Use: February 1 + 266 days.
The resulting date is the estimated due date.
Gestational age would still be expressed using the standard pregnancy dating system, which places the start of the gestational timeline approximately two weeks before conception.
IVF Pregnancy Due Date Example
Worked Example
Suppose a Day 5 blastocyst was transferred on March 1.
Use: March 1 + 261 days.
For a Day 3 embryo transferred on March 1: March 1 + 263 days.
The difference occurs because the Day 5 embryo had already developed two days longer before transfer.
Try It Yourself
First day of LMP: January 1. Average cycle: 28 days.
Question: About how many days are added to estimate the due date?
Answer: 280 days.
Known conception date: February 1.
Question: About how many days are added?
Answer: 266 days.
Pregnancy Due Date vs. Pregnancy Week
"Pregnancy due date" and "pregnancy week" describe two different things.
The pregnancy due date is the estimated calendar date corresponding to approximately 40 weeks of gestational age.
The pregnancy week tells you where you are on that timeline today.
Someone can therefore use this calculator even if the due date is already known. Enter the provider-given due date and the calculator can estimate current gestational age, current trimester, approximate conception date, and key points on the pregnancy timeline.
Why Two Pregnancy Calculators Can Give Different Dates
Small differences between pregnancy calculators can happen for several reasons.
One tool may assume every cycle is exactly 28 days.
Another may adjust for cycle length.
One may start counting pregnancy weeks differently in its display.
A user may accidentally enter the end of the last period instead of the first day.
An IVF calculator may incorrectly treat transfer day like conception day.
And an official clinical due date may have been established or confirmed by an early ultrasound rather than by the LMP alone.
That is why the calculation method should always be visible alongside the result.
CalculateThisWay should show exactly what information was used and how the date was calculated.
Your Pregnancy Timeline Is More Useful Than One Date
A due date is useful, but a pregnancy timeline often answers more practical questions.
Once an estimated due date is established, you can calculate:
current gestational age
current pregnancy week
current trimester
estimated conception date
approximate LMP
trimester transition dates
37-week point
full-term period
estimated due date
Displaying those dates together makes the result much easier to understand than showing one large calendar date by itself.
Visual: A Personalized Pregnancy Journey Timeline
Once you calculate your dates above, this same style of timeline fills in with your own estimated conception, trimester transitions, term milestones, and due date.
Milestone points used in the personalized pregnancy journey timeline
Milestone
Gestational Age
Estimated conception
About 2 weeks
End of first trimester
13 weeks 6 days
Start of second trimester
14 weeks
Start of third trimester
28 weeks
Early term begins
37 weeks
Full term begins
39 weeks
Estimated due date
40 weeks
Late term begins
41 weeks
Pregnancy Weight Gain and Other Pregnancy Calculators
Pregnancy dating tells you where you are in the pregnancy timeline, but it does not answer every pregnancy-related question.
For users trying to understand fertile days before pregnancy, see the CalculateThisWay Ovulation Calculator.
Pregnancy Dating Methods at a Glance
Method
What You Enter
Basic Calculation
Best Used When
LMP
First day of last period
+280 days, adjusted for cycle length when appropriate
LMP is known and cycles are reasonably predictable
Known conception
Conception date
+266 days
Conception timing is reasonably known
IVF Day 3
Transfer date
+263 days
Day 3 embryo transfer
IVF Day 5
Transfer date
+261 days
Day 5 blastocyst transfer
Known EDD
Provider-given due date
Calculate backward
EDD has already been established
Ultrasound dating
Scan date plus documented gestational age
Calculate remaining time to 40 weeks
Provider has supplied ultrasound gestational age
Pregnancy Calculator Questions
How do I calculate my pregnancy due date from my last period?
Start with the first day of your last menstrual period and add approximately 280 days. This calculator can also adjust the estimate when the average cycle length differs from 28 days.
How many weeks pregnant am I?
Gestational age is counted from the first day of the last menstrual period or another established dating method. Enter the information you know and the calculator will show completed weeks, additional days, and your current pregnancy week.
What if I already know my due date?
Choose the Due Date mode and enter the date established by your healthcare provider. The calculator can work backward to estimate your current gestational age, trimester, approximate conception date, and pregnancy timeline.
Can I calculate a due date from conception?
Yes. A commonly used estimate is conception date plus 266 days.
How is an IVF due date calculated?
The calculation uses the embryo transfer date and embryo age. A Day 3 transfer adds 263 days and a Day 5 blastocyst transfer adds 261 days.
What is the difference between gestational age and fetal age?
Gestational age follows the medical convention of counting from the last menstrual period. Approximate fetal or conception age starts around fertilization and is generally about two weeks less in a typical cycle.
Why does pregnancy start before conception?
The dating system was created around the menstrual cycle because the first day of the last period is usually easier to identify than the exact moment of fertilization.
Is a pregnancy due date exact?
No. It is an estimated reference date used to track pregnancy timing. Birth does not need to occur on the exact EDD.
Can irregular periods affect a due date calculation?
Yes. LMP calculations assume predictable cycle timing. When cycles are irregular or the LMP is uncertain, healthcare professionals may rely more heavily on early ultrasound dating.
Which due date should I use if my doctor gives me a different date?
Use the due date established by your healthcare professional for medical care. An online pregnancy calculator is educational and should not replace clinical dating.
This calculator provides educational estimates and does not replace prenatal care or the due date established by your healthcare professional.