Pregnancy Conception Calculator

Use this free pregnancy conception calculator to estimate when you conceived and when your baby is due. Enter the first day of your last menstrual period (LMP), your average cycle length, or an ultrasound date. The calculator returns an estimated conception date, estimated due date, and how many weeks pregnant you are today.

These results are estimates. Only your healthcare provider can confirm your dates with a clinical exam or ultrasound scan.

Pregnancy Due Date Calculator: Estimate Your Conception Date and Due Dates

This pregnancy due date calculator works in two directions. It estimates the date of conception based on your cycle, and it projects your due date forward from there.

You can enter one of three starting points:

• First day of your last menstrual period (LMP) and your cycle length
• Ultrasound date with the gestational age measured at that scan
• IVF transfer date and embryo age (Day 3 or Day 5)

The calculator then estimates your conception date, your estimated due date, your current week of pregnancy, and your trimester. Most people start with their LMP because it is the easiest date to remember.

Want to know your conception date? The tool works backward from your due date, too. If you already know your due date from a prenatal visit, enter it and the calculator will estimate when fertilization likely occurred.

How Your Due Date Is Calculated From Your Last Period

The standard due date calculation starts from the first day of your last menstrual period. It adds 280 days (40 weeks) to that date. This method is called Naegele's rule, and it assumes a regular 28-day cycle with ovulation on day 14.

Here is the math in steps:

1. Start with the first day of your last period.
2. Subtract 3 months.
3. Add 7 days.
4. Add 1 year.

That gives you your estimated due date. The same result comes from simply counting 280 days forward from your LMP.

This formula works best when your menstrual cycles are roughly 28 days. If your cycle is shorter or longer, ovulation shifts earlier or later, and so does your estimated due date. The calculator adjusts for this when you enter your actual cycle length.

Conception Date and Gestational Age

Gestational age counts pregnancy from the first day of your last period, not from the day you actually conceived. That means gestational age includes about two weeks before fertilization even happened.

So when a provider says you are "6 weeks pregnant," the embryo is closer to 4 weeks old. This can be confusing, but it is the standard way pregnancy is dated worldwide.

Gestational Age vs. When You Conceive

Gestational age and conceptional age (also called fetal age) are two different clocks.

The date of conception is not always the same as the day you had sex. Sperm can survive in the reproductive tract for up to five days. Fertilization happens when sperm meets egg, which could be days after intercourse.

Because pinpointing the exact date of conception is difficult, healthcare providers default to gestational age for prenatal care, ultrasound measurements, and due date calculation.

Estimated Due Date Based on Last Menstrual Period and Cycle Length

A standard 28-day cycle assumes ovulation around day 14. If your cycle is 32 days, ovulation likely happens around day 18. That four-day shift pushes your estimated due date four days later.

The calculator accounts for this. Enter your average cycle length and it adjusts the conception window and due date accordingly.

Here is how cycle length affects the estimate:

• 25-day cycle: Ovulation near day 11. Due date shifts about 3 days earlier than the standard calculation.
• 28-day cycle: Ovulation near day 14. Standard Naegele's rule applies.
• 35-day cycle: Ovulation near day 21. Due date shifts about 7 days later.

If you are unsure of your average cycle length, 28 days is the default. Your provider may refine the date later with an ultrasound scan.

Using an Ultrasound Scan to Estimate Your Due Date

Ultrasound dating measures the size of the embryo or fetus and compares it to known growth curves. In early pregnancy, embryos grow at a very consistent rate, so an ultrasound between 6 and 9 weeks is the most accurate way to date the pregnancy.

A first-trimester ultrasound is accurate to within about 5 to 7 days. A second-trimester scan is accurate to within about 10 to 14 days. Later scans become less reliable for dating because growth rates vary more between babies.

Early Pregnancy Ultrasound and Due Date Change

Your due date may be adjusted after an early ultrasound if it differs from the LMP-based estimate by more than a set threshold. Common clinical guidelines suggest:

• First trimester (up to 13 weeks): Adjust if the ultrasound date differs by more than 5 to 7 days.
• Second trimester (14 to 20 weeks): Adjust if the difference exceeds 10 to 14 days.
• After 20 weeks: Ultrasound dating is generally not used to change the due date.

A due date change does not mean something is wrong. It usually means ovulation happened earlier or later than the LMP calculation assumed. Once your provider sets a final due date, it typically stays fixed for the rest of pregnancy.

Conception Calculator for Irregular Periods

When your menstrual cycles are irregular, the LMP method becomes less reliable. If you do not ovulate on a predictable schedule, the standard 280-day formula can be off by a week or more.

If you have irregular periods, consider these approaches:

• Track ovulation signs. Basal body temperature, cervical mucus changes, or ovulation predictor kits can help narrow the window.
• Use an early ultrasound. This is often the most accurate way to date the pregnancy when cycle length varies.
• Enter your best estimate. If you know your approximate cycle length over the past few months, enter the average. The calculator will adjust.

When did I conceive if I have irregular periods? Honestly, a single calculation cannot answer that with precision. An early pregnancy ultrasound gives your provider the best information to accurately date the pregnancy.

IVF Transfer Date and Estimated Due Date

If you conceived through IVF, you know the exact date the embryo was transferred. This makes due date calculation more straightforward.

The formula depends on the embryo stage at transfer:

• Day 3 embryo transfer: Due date = transfer date + 263 days (or subtract 3 days, then add 280 days from the adjusted LMP equivalent).
• Day 5 embryo transfer (blastocyst): Due date = transfer date + 261 days.

In both cases the calculator works backward to assign an equivalent LMP date, then counts 40 weeks forward. Enter your IVF transfer date and embryo type (Day 3 or Day 5) and the tool does the rest.

Because IVF removes the guesswork around ovulation, estimated due dates from transfer dates tend to be quite accurate. Your fertility clinic may still adjust the date slightly after a confirmation ultrasound.

How Due Dates, Ovulation, and Fertilization Connect

Your due date, ovulation day, and fertilization date are all linked, but they are not the same moment.

Here is the typical pregnancy timeline for a 28-day cycle:

1. Day 1: First day of your last menstrual period.
2. Around day 14: Ovulation. The ovary releases an egg.
3. Day 14 to 15: Fertilization, if sperm is present. This is your estimated conception date.
4. Day 20 to 24: The fertilized egg implants in the uterine lining.
5. Day 280 (week 40): Estimated due date.

Ovulation is the trigger. Without it, conception cannot happen. The due date is simply 266 days (38 weeks) after conception, or 280 days (40 weeks) after the LMP.

When You Conceive: Sperm, Egg, and the Fertility Window

The fertility window spans about six days: the five days before ovulation plus the day of ovulation itself. Sperm can survive in the reproductive tract for up to five days, but the egg is viable for only about 12 to 24 hours after release.

This means you could have intercourse on a Monday, ovulate on a Thursday, and conceive on Thursday. The conception date would be Thursday, not Monday.

Because of this overlap, the exact date of conception is hard to pinpoint unless you are tracking ovulation closely. The calculator uses your cycle length to estimate the most likely ovulation day, then marks that as the probable conception date.

Pregnancy Calculator Estimates, Not Exact Dates

This pregnancy conception calculator provides estimates. It cannot tell you the exact day you conceived or the exact day your baby will arrive.

Key reasons results are approximate:

• Ovulation does not always happen on the predicted cycle day.
• Sperm survival creates a multi-day conception window.
• Every pregnancy progresses at a slightly different rate.
• Only about 4% of babies are born on their exact due date.

Use these results as a planning guide. Discuss them with your healthcare provider at your first prenatal appointment. They can confirm or adjust your dates using clinical examination, ultrasound measurements, and your medical history.

This calculator is not a substitute for prenatal care, diagnosis, or medical advice.

Free Pregnancy Due Date Calculator for Every Trimester

Pregnancy is divided into three trimesters. Knowing your estimated due date helps you map out the full pregnancy timeline.

Use this free pregnancy due date calculator at any point. Enter your LMP, ultrasound date, or known due date and the tool shows where you fall in the timeline right now.

How Many Weeks Pregnant: 40 Weeks From Your Last Period

Full-term pregnancy is defined as 40 weeks from the first day of your last period. That is 280 days, or roughly 9 calendar months and 1 week.

Here is how the weeks break down:

• Early term: 37 weeks 0 days through 38 weeks 6 days
• Full term: 39 weeks 0 days through 40 weeks 6 days
• Late term: 41 weeks 0 days through 41 weeks 6 days
• Post-term: 42 weeks 0 days and beyond

Most babies arrive between 39 and 41 weeks. Your due date is the midpoint of a normal delivery window, not a deadline.

How many weeks pregnant am I today? Enter your dates into the calculator and it shows your current week and day count.

Prenatal Care and Why Your Due Date May Change

An accurate due date matters because it guides your entire prenatal care schedule. Screening tests, growth assessments, and delivery planning all depend on knowing how far along you are.

Your due date may change for several reasons:

• An early ultrasound shows a different gestational age than the LMP estimate.
• Your initial LMP date was uncertain or based on irregular periods.
• A first-trimester scan reveals the embryo is measuring ahead or behind.

A due date change does not mean a problem with the pregnancy. It means your provider has better data. Once a final due date is set (usually by the end of the first trimester), it rarely changes again.

Bring your calculator results to your first prenatal appointment. They give your provider a useful starting point. From there, your care team will confirm the dates and build a prenatal care plan tailored to you.