Pregnancy Due Date Calculator

What a due date really is, and why the count starts before conception

An estimated due date, or EDD, is the midpoint of a window, not an appointment. The standard way to work it out is Naegele’s rule: take the first day of your last menstrual period, add 280 days and you have 40 weeks of pregnancy. Clinics count the same way, which is why your notes say something like 24w3d rather than a number of months.

The part that confuses almost everyone is that the clock starts about two weeks before there is anything to count. Weeks one and two of a pregnancy are the weeks of the period itself and the run up to ovulation. Conception lands around week two, implantation around week four, and a positive home test usually arrives somewhere in week four or five. Counting from the day you think you conceived gives a number roughly two weeks lower than the one your provider will use, and that gap is why a calculator can look two weeks out.

The reason for the convention is practical rather than biological. The first day of a period is something you can see and write down; the day of ovulation is not. Even so, ACOG notes that only about half of women recall that date accurately, which is one reason an early ultrasound is used to confirm or correct it.

Which start date to enter

• Last period, regular cycles: the default. Enter the first day of bleeding, not the last. If your cycles run close to 28 days, this is as good as arithmetic gets.
• Last period, long or irregular cycles: still usable. If your cycles are regular but not 28 days, enter your usual length in the optional cycle box and the calculator moves the date by the difference: 7 days later for a 35 day cycle, 3 days earlier for a 25 day one. It is a rough correction, so read the result as provisional and let an early scan settle it.
• Known conception date: choose the conception option if you tracked ovulation with LH strips or basal body temperature, or if there was a single possible date. The calculator works backward to a notional last period and then forward again.
• IVF: use the conception option with the egg retrieval or fertilization date, not the transfer date, then compare it with the date your clinic gives you. The clinic date wins.
• You have already had a dating scan: use the date your provider recorded. A first trimester scan beats every calculation on this page.

How to find out how many weeks pregnant you are

1. Leave the start option on last menstrual period, which is what clinicians use.
2. Enter the first day of that period. If you are unsure by a day or two, enter your best guess and treat the result as approximate.
3. If your cycle is not 28 days, enter its usual length, counted from the first day of one period to the first day of the next. Leave it blank for 28. The due date, the weeks and the milestones all move together.
4. Read the three cards: how far along you are in weeks and days, which trimester that falls in, and how many days are left to the estimated date.
5. Check the timeline below them. It shows where you are between week 0 and week 40, with the usual points of interest marked.
6. From week 14, the weekly size panel shows the median estimated weight for your current week, the range 8 in 10 babies fall within, and a fruit or vegetable that weighs about the same. Before week 14 there are no numbers and no comparison. Expand the chart to see weeks 14 to 40 at once.

How to date from conception or an IVF transfer

1. Switch the start option to conception date and enter the day of ovulation, fertilization or egg retrieval.
2. The calculator adds 266 days, which is the same 280 day estimate minus the two weeks that precede conception in the standard count.
3. Do not enter an embryo transfer date: the calculator would add 266 days to it and land three to five days late. ACOG’s guidance counts 261 days from the transfer of a day 5 embryo and 263 days from the transfer of a day 3 embryo, which is the same date as retrieval plus 266.
4. Whichever date you end up with, keep using the same one. Switching between a period based date and a scan based date halfway through makes every later measurement harder to interpret.

How the “about as heavy as” produce is picked

From week 14, each week is paired with a fruit or vegetable whose whole weight, as you would bring it home from the store, is within 20 percent of that week’s WHO median. At 17 weeks the median is 179 g and a medium banana with its peel weighs about 184 g, so week 17 gets a banana.

Most of the weights come from USDA FoodData Central. USDA lists the edible part only, so a medium banana appears as 118 g, which is the peeled weight. The same USDA database also records how much of each food is peel, core, seeds, rind or outer leaves (36 percent for a banana), and dividing the edible weight by what is left gives the whole item: 118 ÷ 0.64 ≈ 184 g. For broccoli, cauliflower and a mini watermelon, USDA has no single-item weight or the arithmetic gives something nobody would recognize from a store, so those weights come from variety descriptions published by the Taiwan Agricultural Research Institute, using the midpoint where a range is given.

There is no comparison before week 14, because there are no weights to compare yet. Two later weeks are also blank. At week 34 the median is 2,312 g: a pineapple is more than 20 percent lighter and a mini watermelon more than 20 percent heavier. At week 40 the median is 3,617 g, and the heaviest item on the list, the mini watermelon at about 2.8 kg, is more than 20 percent short of it. Rather than stretch the rule, those weeks are left blank.

This page used to follow the fruit-a-week sequence that circulates on many pregnancy sites. Those comparisons were picked by size, came with no source, and once the WHO weights went in they put a banana next to a 330 g baby at week 20, so the whole set was replaced. Produce varies in size, so treat the match as a rough feel for the weight, nothing more.

Important limits

This calculator is not medical advice and cannot replace care from a doctor or midwife. It applies a formula to a date you typed. It does not know your cycle, your history, whether you are carrying more than one baby, or anything a scan would show. Confirm your date with your own provider, and contact them rather than a web page about any symptom that worries you.

The milestone list on this page is generic. Prenatal visit schedules are set nationally and differ a great deal: the number of funded visits, when screening is offered and which vaccines are given all depend on where you are. Use the list as a rough map of the pregnancy, and use your own clinic’s schedule for actual appointments.

The weights in the size panel come from the World Health Organization fetal growth charts published in 2017. That study followed 1,387 low-risk pregnancies in ten countries and calculated an estimated fetal weight from ultrasound measurements of the head, abdomen and thigh bone, so these are estimates, not weighed babies. The page shows the median for each week and the 10th to 90th percentile range, which by definition leaves one baby in ten below it and one in ten above it. The WHO chart starts at week 14 and has no length measurement, so there are no numbers before week 14 and no lengths at any week. Your scans and your provider are what tell you whether growth is on track.