Your pregnancy age and due date will appear here as you complete the fields.
Choose a week to explore what is developing, how size is described, and what the numbers can tell you.
Illustration of this stage · not to scale
Muscles and internal organs are continuing to grow. Tissue in the developing skeleton is beginning to harden into bone, while the body remains curled. The length shown here runs from the crown to the bottom.
Your estimated due date is a reference for care, rather than a prediction of the exact birthday.
The NHS guide uses crown–rump length, from the top of the head to the bottom, through week 19. From week 20, it describes length from head to heel. The change includes the legs, which is why the chart has a break even though development is continuous.
A fruit or vegetable comparison gives you an everyday object to picture. Its shape and weight may differ, even when one dimension is similar.
NHS guide to the measurement changeThe line shows the middle of the WHO study’s estimated fetal weights for each week from 14 to 40. The shaded band spans the 10th to 90th percentiles, or the middle 80 percent of the reference distribution.
These values describe a study population. Your care team uses ultrasound measurements and the wider clinical picture to assess individual growth.
WHO fetal-weight reference, Table 11Approximate weekly lengths. A break marks the measurement change at week 20.
Week 12 reference: 5.4 cm
Reference: NHS week-by-week length information. Values are approximate.
Pregnancy dates can be surprisingly difficult to untangle. You might have a note in your calendar, a scan report, or a due date from your clinic, and still wonder how those pieces fit together. Whether you feel excited, uncertain, or a little of both, you deserve an explanation you can follow at your own pace.
Our Pregnancy Calculator brings your estimated pregnancy age, due date and weekly guide into one place. Start with the information you have. You can explore an example first, come back to your own dates when you are ready, and keep a copy of the result if it helps you prepare for a conversation with your care team.
Try the steps below. Tap a card, explore a method, or clear the example to see what happens. This walkthrough uses its own example dates, so you can have a look around without changing your result.
The calculator starts with empty fields. Choose Try an example if you would like to explore a labelled sample first. Choosing Use my dates then clears the sample and leaves the period date and cycle length for you to select.
You do not need every kind of pregnancy record. Next, choose the starting point that matches the information you have.
The result uses gestational age, the dating convention used in pregnancy care. It is different from the time since conception. A period-based estimate starts with 280 days from the last period, with an approximate cycle adjustment in this tool. You can read more in the NHS explanation of due-date calculations (opens in a new tab).
A result of 12 weeks + 3 days means 12 complete weeks and another 3 days on the date shown. Our weekly guide uses completed weeks, so it shows the week-12 reference until the estimate reaches 13 weeks. The “As of” date is today; it updates when the day changes or you return to the tab.
You do not need to convert the result into months to use the guide. Weeks and days keep the meaning consistent between your result, the slider and the timeline. Once you have a result, the Dating method and assumptions section below shows the exact formula for your selected method.
There is no need to take in every number at once. These are the details you will see:
The progress bar places the estimate along a 40-week timeline. It does not show how much development is “finished” or tell you when labour will begin. If your entered dates put today before the estimated pregnancy start, the result says so instead of showing a negative week count as a baby’s stage.
The slider lets you explore weeks 4–41. Moving it changes the week you are reading about; it does not change your entered dates or pregnancy result. If you wander ahead or look back, “Back to your week” returns you to the week associated with your result.
The fruit or vegetable is a familiar way to picture an approximate size. It is an illustration, not a life-size image or a measurement of your baby. The small comparison in your result stays tied to your calculated week. Selecting Learn more takes you to that same week in the guide.
The length references come from the NHS weekly guide. Before week 20, the reference is head to bottom; from week 20 it is head to heel. That change explains the break in the length chart. It should not be read as a sudden jump in growth. The NHS week-20 guide explains the measurement change (opens in a new tab).
The weight panel uses the published WHO fetal-growth reference (opens in a new tab) for weeks 14–40. Its median and chart band describe a study population, not your baby’s measured weight or personal percentile. Where a selected reference does not provide a value, the guide leaves it unavailable. Metric and Imperial change the units, using the same underlying reference values.
The timeline turns the chosen estimate into calendar milestones, including the trimester changes, term windows and estimated due date. It is there to help you see how the dates fit together. These are calendar markers, not booked appointments or a recommendation about when to give birth.
If your care team gives you a confirmed dating reference or a new care plan, use that information when making practical plans. The calculator cannot schedule your scans, choose a birth date or decide which monitoring you need.
Seeing two different estimates can be unsettling. A period date may be difficult to remember, particularly with irregular cycles, and a cycle adjustment cannot confirm when ovulation happened. If you are unsure which date to use, a dating report or a conversation with your midwife or clinician can help. You do not need to settle conflicting reports on your own.
If a clinician has already established your due date, choose Known due date. The advanced comparison also retains an established date and gives IVF timing priority when entered. It explains discrepancies for discussion; it does not make a clinical decision to replace an agreed date. ACOG recommends reserving subsequent due-date changes for rare circumstances (opens in a new tab).
You can use the calculator without creating an account. The controls give you a few different ways to keep or revisit the information:
Calculator entries are cleared when you reload. You can open your saved account records from the calculator.
This calculator can help you organise dates and understand the information on screen. It cannot confirm a pregnancy, assess your baby’s health, interpret a scan or predict an exact birth date. A result that looks reassuring is not a reason to put off advice about a symptom that worries you.
Bring your dates and reports to your care team, and ask which due date to use if anything feels unclear. You deserve to understand your own care, and it is always reasonable to ask for an explanation.
Return to the calculatorA due date is an estimate. Your care team can confirm the dating and explain what it means for you. Period-based estimates can be less reliable when cycles are irregular or the period date is uncertain; use your dating ultrasound report or an established clinical due date when available.
Weekly development and size information describe general references. They do not confirm your baby’s stage, measure growth or replace individual clinical assessment.
Understand pregnancy weeks, typical development and the information you see in the guide.
Gestational age conventionally starts from the last menstrual period. In a typical 28-day cycle, conception occurs around two weeks later. Actual ovulation can vary, so the estimated start is not proof of a conception date.
Source informationA period-based estimate may be less reliable. You can instead enter the gestational age and date on a dating ultrasound report, or the due date established by your clinician. The comparison mode also lets you mark menstrual dates as uncertain.
Source informationNo. The development guide shows approximate weekly length references and a published fetal-weight reference. Your own baby’s measurements require clinical assessment. No personal growth percentile is calculated from your period or due date.
Source informationThe reference changes from head-to-bottom measurements to head-to-heel measurements. Those are different dimensions, so the graph deliberately leaves a break between weeks 19 and 20 instead of drawing a misleading growth line.
Source informationThe calculator runs in your browser. Your entries clear when you reload. If you sign in and choose Save Record, the dating details and estimate are stored in your PeriodlyWise account. You can also download or print a copy.
Use the calculator to learn about pregnancy dating. Your healthcare professional can confirm the due date to use for your care and interpret any ultrasound findings or symptoms.
The calculator uses published dating guidance and separate references for typical growth and development.
LMP, ultrasound and assisted-reproduction dating. The comparison preserves established dates and flags situations needing clinical review.
ACOG · Term-pregnancy definitionsOriginal development notes with cited numerical references. WHO weights are population reference values, not individual predictions.
Calculations run in your browser. Your inputs clear when you reload. If you choose Save Record while signed in, your estimate is kept in your account until you delete it.
Contains public sector information licensed under the Open Government Licence v3.0. WHO reference data: Kiserud et al. (2017), PLOS Medicine, used under Creative Commons Attribution. The stage artwork is illustrative and uses representative images across several weeks. Source links do not imply endorsement.
The Pregnancy Calculator is for general education. It estimates how far along you may be and when your baby may be due based on the information you enter. The development illustrations and size references describe typical stages of pregnancy.
The calculator cannot confirm pregnancy, assess your baby’s growth or diagnose complications. Follow your healthcare professional’s advice and use the due date they establish when making decisions about your care.
Read PeriodlyWise’s full Medical DisclaimerNo valid result has been calculated.
Educational estimate. Confirm dating with your healthcare professional. References: ACOG Committee Opinion 700; NHS weekly length information; Kiserud et al. 2017 WHO fetal-growth reference.
