Pregnancy Calculator — All-in-One

Pregnancy Calculator: Find Your EDD, Weeks & Timeline

How to Use Pregnancy Calculator — All-in-One

1. Step 1: Choose a dating method

   Select LMP, Conception, Ultrasound, IVF, or Due Date depending on what information you have.

2. Step 2: Enter your date

   Type the first day of your last period, conception date, ultrasound scan date, IVF transfer date, or your known EDD.

3. Step 3: Adjust cycle length (optional)

   If using LMP, change the average cycle length from 28 days to match your cycle for a more accurate EDD.

4. Step 4: Review instant results

   Your EDD, gestational age, trimester, baby size, and milestone timeline update automatically as you enter dates.

5. Step 5: Change the as-of date

   Set a past or future date to see what your gestational age will be on that specific day.

6. Step 6: Save or share

   Copy the pregnancy summary to clipboard or download the EDD as a calendar event (.ics file).

Key Features

• EDD from LMP, conception, ultrasound, IVF, or known due date
• Gestational age in weeks and days for any date
• Trimester tracking with color-coded progress bar
• Week-by-week baby size comparison
• Milestone timeline with 10 key pregnancy dates
• Add EDD to calendar and copy pregnancy summary

Understanding Results

Formula

The pregnancy calculator applies standard clinical rules to estimate key dates. By default (LMP method), it uses Naegele’s rule: the estimated due date (EDD) is the first day of your last menstrual period plus 280 days (40 weeks). If your average cycle length differs from 28 days, the tool shifts the EDD by the difference so longer cycles move the due date later and shorter cycles move it earlier. When you select Conception, the calculator assumes clinical gestational age starts 14 days before conception and sets EDD to conception + 266 days. With an Ultrasound measurement, you enter the scan date and the gestational age read on the report (weeks + days). The tool back‑dates the clinical LMP from that measurement and projects the EDD 280 days from that computed LMP. The IVF method accounts for embryo age at transfer (day‑3 or day‑5) and uses the standard EDD formula for IVF: EDD = transfer + (266 − embryo age in days).

Reference Ranges & Interpretation

Gestational age is reported in weeks and days as of the date you choose (default is today). Trimesters follow common cutoffs: first trimester through 13 weeks 6 days; second trimester 14 weeks 0 days through 27 weeks 6 days; third trimester starts at 28 weeks. The timeline in your results highlights key points many clinicians discuss, such as the end of the first trimester (~14 weeks), the anatomy scan (~20 weeks), glucose screening (24–28 weeks), and the start of the third trimester (28 weeks). “Full term” is commonly considered 39–40 weeks. While many pregnancies deliver around the EDD, normal spontaneous birth may occur before or after this date.

Dating methods have different accuracy. LMP assumes regular ovulation roughly 14 days before the next period. Early first‑trimester ultrasound is often the most precise way to date pregnancy because crown‑rump length has a narrow normal range in early weeks. If an ultrasound due date differs from LMP by more than a small margin, clinicians may adopt the ultrasound date. These conventions vary by organization and gestational age at the time of scan.

Assumptions & Limitations

Results are for planning and education only and do not replace clinical care. Irregular cycles, recent hormonal contraception, breastfeeding, or uncertain dates can all reduce LMP accuracy. Ultrasound‑based estimates depend on the scan date and measurement quality. IVF timing depends on precise transfer details. If you are unsure about your dates, have symptoms that concern you, or are in a high‑risk pregnancy, talk with your clinician. For general background, see guidance from professional organizations such as ACOG and WHO. External links open in a new tab: ACOG due date methods and WHO recommendations.

Complete Guide: Pregnancy Calculator — All-in-One

A pregnancy calculator takes one piece of information you already have—your last period, a scan measurement, an IVF transfer date, or even just the EDD your provider gave you—and turns it into the full picture: estimated due date, gestational age in weeks and days, current trimester, and a timeline of milestones ahead. The tool above does all five methods in one place so you don't need separate calculators for each scenario.

Most searches that land on this page ask the same question in different ways: “If my EDD is September 10, how many weeks pregnant am I today?” The answer depends on simple arithmetic—but the arithmetic involves counting backward from your due date to a theoretical LMP, then forward to today. That's exactly what this calculator automates, instantly and privately in your browser.

How pregnancy dating actually works

Clinical pregnancy dating doesn't start at conception. It starts at the first day of your last menstrual period (LMP), roughly two weeks before ovulation. That means when a provider says “you're 8 weeks pregnant,” the embryo is closer to 6 weeks old. This convention exists because LMP is a date most people can pinpoint, while the exact moment of fertilization is usually unknown.

From the LMP, clinicians apply Naegele's rule: add 280 days (40 weeks) to get the estimated due date. The formula was published in 1812 and remains the default because it's simple, widely taught, and close enough for the majority of pregnancies with regular 28-day cycles. When cycles are longer or shorter, you shift the EDD accordingly—a 32-day cycle adds 4 extra days, a 25-day cycle subtracts 3.

Five dating methods compared

Not every pregnancy starts with a known LMP. The table below compares the five approaches this pregnancy calculator supports, along with how each derives the EDD.

IVF is the most precise because the transfer date is documented to the day. Early ultrasound comes next—crown-rump length between 6 and 10 weeks has a tight normal range, so the margin of error is small. LMP is the least precise, especially with irregular cycles, but it's the most commonly available starting point.

Pregnancy calculator worked example: LMP to EDD step by step

Suppose your last period started on January 15, 2026, and your average cycle is 30 days.

1. Start with LMP: January 15, 2026.
2. Apply Naegele's rule: January 15 + 280 days = October 22, 2026.
3. Adjust for cycle length: 30 − 28 = +2 days → EDD = October 24, 2026.
4. Estimate conception: LMP + (30 − 14) = LMP + 16 = January 31, 2026.
5. Check today's GA: If today is April 11, 2026, that's 86 days since LMP − cycle adjustment, giving about 12 weeks 2 days—end of the first trimester.

The calculator performs these steps instantly when you enter January 15 and set cycle length to 30. You'd see your EDD, the 12w2d gestational age, and that you're about to enter the second trimester.

How to find your EDD from weeks pregnant

If your provider told you that you're 16 weeks and 3 days pregnant on March 20, you can reverse-engineer the EDD. Gestational age of 16w3d equals 115 days. A full pregnancy is 280 days. So 280 − 115 = 165 days remaining, and March 20 + 165 days = September 1—that's the estimated due date.

Conversely, if you already know your EDD and want to figure out how many weeks pregnant you are right now, the calculator does this for you with the “Due Date” method. Enter your EDD, and it instantly shows your current gestational age. This is the most common question from Bing search data: “If my EDD is [date], how many weeks pregnant am I?”

When does each trimester start and end?

Trimester boundaries aren't universally standardized, but most organizations (ACOG, NHS) use these cutoffs:

The pregnancy calculator color-codes your progress bar by trimester—blue for the first, amber for the second, and green for the third—so you can see at a glance where you are and when each transition happens.

What affects EDD accuracy

Only about 4% of babies arrive exactly on their due date. The EDD is the midpoint of a bell curve, and most deliveries fall within a two-week window on either side. Several factors shift the estimate:

• Irregular cycles: If your cycle varies between 26 and 35 days, LMP-based dating can be off by a week or more. Switching to ultrasound or conception-based dating helps.
• Late vs. early ultrasound: A scan at 8 weeks is accurate within 3-5 days. By 20 weeks, the margin widens to 7-10 days because fetal size varies more as pregnancy progresses.
• Multiple pregnancies: Twins and higher-order multiples often deliver earlier. The EDD formula is designed for singletons.
• First pregnancy vs. subsequent: First-time mothers tend to deliver slightly later than the EDD on average. Second and later pregnancies may deliver a few days earlier.

When an ultrasound-based EDD disagrees with the LMP-based date by more than 5-7 days (depending on gestational age at scan), ACOG recommends using the ultrasound date. Your provider may “redecide” your EDD after an early scan—that's a routine, evidence-based practice, not a mistake.

Baby size week by week

The calculator shows a fruit-size comparison for your current week. Here's a sampling across all three trimesters:

These are averages from published growth charts. Individual babies vary, especially in the third trimester where genetics and maternal factors play a bigger role. The comparison is meant to give you a tangible sense of scale, not a diagnostic measurement.

Common mistakes when calculating gestational age

A few pitfalls trip people up when they try to count weeks manually:

1. Counting from conception instead of LMP. This gives a number two weeks lower than the standard. If you conceived 10 weeks ago, your gestational age is 12 weeks, not 10. The pregnancy calculator handles this conversion automatically.
2. Ignoring cycle length. A 35-day cycle means ovulation likely happened around day 21, not day 14. Using a 28-day assumption overestimates gestational age by a full week.
3. Confusing “weeks” with “the Nth week.” At 12 weeks 0 days, you are in week 13 (the 13th week of pregnancy). The notation “12w0d” means 12 full weeks have passed.
4. Using the wrong ultrasound measurement. Later scans (after 20 weeks) can differ from earlier dating by over a week. Always prefer the earliest available scan for EDD.

What to do with your results

Once you have your EDD and gestational age, use the milestone timeline to plan ahead. The anatomy scan around 20 weeks, the glucose screening around 24-28 weeks, and the full-term window starting at 37 weeks are dates worth marking on your calendar. The “Add EDD to calendar” button downloads a .ics file you can import into Google Calendar, Outlook, or Apple Calendar.

If you need a focused tool for a single question, try the due date calculator (EDD) for a streamlined due date estimate, or the pregnancy week calculator to see exactly which week you're in. For tracking ovulation and cycle patterns before pregnancy, the ovulation calculator and conception date calculator are useful companions.

For trimester-specific date boundaries, the trimester calculator gives a clear view. And if you're early and wondering when a home test will be reliable, the pregnancy test calculator estimates the earliest date based on your cycle.

References

1. American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date. Committee Opinion No. 700, 2017.
2. Naegele FC. Lehrbuch der Geburtshilfe, 1812. Historical basis for the 280-day rule.
3. World Health Organization. WHO recommendations on antenatal care. 2016.
4. Salomon LJ, et al. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound Obstet Gynecol. 2019;53(6):715-723.

This pregnancy calculator is for planning and education. It does not replace clinical assessment. If you have concerns about your dates, symptoms, or pregnancy health, consult your healthcare provider.

Frequently Asked Questions