Online gestogram: pregnancy wheel and due date [Calculator]
Gestograma online
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Let me tell you something: the first time I saw a gestogram, I thought it was some kind of secret decoder. My gynaecologist spun that cardboard disc while she asked when my last period had started, and two seconds later she said, “you’re six weeks and four days”. No app, no screen, just a wheel. The online version at the top of this page does the same thing with your dates, and it also tells you the trimester you’re in and your estimated due date. ❤️
What nobody explained to me back then is that the wheel is only as good as the date you feed it. So let’s look at what it really calculates, what it can’t, and why your app, your wheel and your doctor sometimes give you three slightly different numbers.
Pregnancy wheel calculator: what it actually tells you
A gestogram — also called a pregnancy wheel, gestational disc or obstetric wheel, depending on who you ask — is a circular chart with two concentric discs. The outer one is a calendar (the twelve months); the inner one carries the pregnancy weeks, and on some models the baby’s average measurements too.
When you line up the arrow on the inner disc with the first day of your last period (your LMP), the wheel shows you at a glance:
- your current pregnancy week
- your estimated due date (EDD)
- the trimester you’re in
- on clinical models, average fetal measurements such as crown-rump length, femur length or estimated weight
It is, in essence, a manual pregnancy week calculator: same maths, no screen. And because it assumes a textbook cycle, the number on the wheel is a starting point, not a verdict.
Gestational age vs fetal age: the two-week trick
Before you read your result, one thing worth knowing. The number the wheel gives you is gestational age, and that count starts on the first day of your last period — roughly two weeks before you ovulated. Your baby’s actual age, counted from conception, is called fetal age (or embryonic age).
In practice: at 8 weeks of gestational age, your baby has existed for about 6 weeks. That gap is why the same pregnancy can be “8 weeks” on your notes and “6 weeks” in a book about embryo development. Both are right; they’re counting from different starting lines.
Due date: four ways to get there, and which one your clinic uses
There is no single way to date a pregnancy. These are the four your midwife or gynaecologist may use, with what each one needs and how much to trust it:
| Method | What you need | How it’s calculated | How precise it is |
|---|---|---|---|
| Last menstrual period (Naegele’s rule) | First day of your last period | LMP + 280 days (40 weeks) | Good with regular 28-day cycles; less reliable if your cycle is irregular or long |
| Known ovulation or conception date | The day you ovulated (ovulation test, charting, insemination) | Conception + 266 days (38 weeks) | Very good, but only if you truly know the day |
| IVF or embryo transfer | Transfer date and embryo stage | The clinic adds the days the embryo had already spent in the lab to the transfer date | The most precise method available |
| First-trimester ultrasound | A scan, ideally between 11 and 13 weeks | Measured from the baby’s crown-rump length | The reference method: ACOG recommends using the scan when it differs from the LMP estimate |
The wheel you see here uses the first row. So if your cycle is anything but textbook, read the next three sections before you trust the date.
If your cycle is longer or shorter than 28 days
Naegele’s rule secretly assumes you ovulated on day 14. If your cycles run to 35 days, you probably ovulated around day 21, and a due date calculated from your period will land about a week early. With short 24-day cycles it tends to land late.
The honest fix is simple: add or subtract the difference. Longer cycle than 28 days? Add those extra days to the due date. Shorter? Take them off. Even better, tell your midwife your usual cycle length — she’ll do it for you, and the first scan will settle it.
If you don’t remember your last period
It happens more often than you’d think, especially after coming off the pill, while breastfeeding, or in the middle of a chaotic month. Two options work:
- Count back from what you do remember — the week you had unprotected sex, a positive ovulation test, or the day you felt ovulation pain. It’s approximate, but it narrows the window.
- Let the first-trimester scan date you. Measuring the baby in early pregnancy is more accurate than any calendar guess, which is exactly why dating scans exist.
Either way, don’t spend the first trimester doing arithmetic at 2 a.m. One scan and the question is closed.
If you got pregnant with IVF or a frozen embryo transfer
Here the date isn’t a guess at all: your clinic knows the exact day the embryo was transferred and how many days it had already been growing. A day-5 blastocyst transfer, for example, puts gestational age at 2 weeks and 5 days on transfer day, and the due date is calculated from there.
That means an IVF due date usually beats the wheel, and it also beats a scan performed later on. If you’re pregnant after treatment, use the date your clinic gave you.
Weeks to months: the table people keep asking for
“So how many months is that?” is possibly the most repeated question of every pregnancy. One month of pregnancy isn’t four weeks — it’s about 4.3 weeks — which is why the conversions never look tidy:
| Weeks of pregnancy | Month | Trimester |
|---|---|---|
| 1 to 4 | 1st month | First |
| 5 to 8 | 2nd month | First |
| 9 to 13 | 3rd month | First |
| 14 to 17 | 4th month | Second |
| 18 to 22 | 5th month | Second |
| 23 to 27 | 6th month | Second |
| 28 to 31 | 7th month | Third |
| 32 to 35 | 8th month | Third |
| 36 to 40 | 9th month | Third |
A caveat I’d rather give you upfront: published tables disagree by about a week, because calendar months have four or five weeks and every source rounds differently. If you want the exact conversion for your week, use our weeks to months converter, or browse the week-by-week guide to see what’s happening in yours.
When the ultrasound overrides the wheel
The first-trimester scan — usually between 11 and 13 weeks, when the baby is measured from head to bottom (crown-rump length) — is the most accurate dating tool we have, because at that stage all babies grow at almost the same speed.
ACOG’s guidance is clear about when the scan wins: before 14 weeks, a difference of more than 7 days between the scan and the LMP-based estimate is enough to redate the pregnancy. Later on the threshold widens — around 10 days between 16 and 21 weeks, 14 days between 22 and 27, and 21 days from 28 weeks onwards. From that moment, the scan date becomes the official one in your notes.
So if your wheel said 12 weeks and the sonographer said 11 and a half, don’t panic. Nothing went wrong: you simply moved to a more precise ruler.
How to print your own pregnancy wheel
If you like having one in your bag, it takes five minutes:
- Download the gestogram by clicking the image below
- Print the two discs separately, ideally on cardstock or thick paper
- Cut them out carefully, keeping the tip of the arrow intact
- Join them in the centre with a brad or small fastener, tight enough that they turn with a little resistance
- Align the arrow with your LMP and you’re done — weeks on the inner disc, calendar on the outer one
With week 40 of the inner disc lined up against the outer calendar, you’ll see your estimated due date. And without moving anything, look up today’s date on the calendar to read your current week.
Frequently asked questions about the gestogram
How accurate is a pregnancy wheel for the due date?
It’s a good estimate, not a prediction. It assumes a 28-day cycle and a 40-week pregnancy, and only a small percentage of babies are born exactly on their due date — most arrive between week 37 and week 42. The first-trimester ultrasound is more accurate.
Can I use a pregnancy wheel if my cycles are irregular?
You can, but treat the result as a rough window. With irregular cycles, the LMP tells you very little about when you ovulated, so the dating scan is the way to go.
What’s the difference between gestational age and fetal age?
Gestational age counts from the first day of your last period; fetal age counts from conception. They differ by about two weeks, which is why a book might describe your “6-week embryo” when your notes say 8 weeks.
Why does my app give a different due date from this calculator?
Because they may use different starting points: LMP, conception, or a scan date you entered. A one or two-day difference is normal; if the gap is a week or more, check which date each one is using.
How many weeks is 6 months pregnant?
Around 26 weeks. And 9 months is about 40 weeks — which is why “the ninth month” and “week 40” mean the same thing.
Does the wheel work if I’m expecting twins?
The due date calculation is the same, but twin pregnancies are usually dated and monitored on their own schedule and often end earlier. Follow the plan your doctor sets.
Is the online gestogram as reliable as my doctor’s?
The maths is identical; the difference is the input. Your doctor can add the scan, your cycle history and your medical context, which is why their date is the one that counts.
Sources
To write this article and check how gestational age and the due date are calculated, I’ve used guidance from recognised health organisations and published reviews:
- ACOG (American College of Obstetricians and Gynecologists) — Methods for Estimating the Due Date (Committee Opinion No. 700)
- NHS (United Kingdom) — Due date calculator
- NHS (United Kingdom) — Pregnancy week by week
- World Health Organization (WHO) — WHO recommendations on antenatal care for a positive pregnancy experience
- MedlinePlus (U.S. National Library of Medicine) — Prenatal care
- PubMed (Aust N Z J Obstet Gynaecol, 2021) — Naegele’s rule and the length of pregnancy – A review
- PubMed (Human Reproduction, 2013) — Length of human pregnancy and contributors to its natural variation
Note: the fetal measurements that some gestograms include (weight, length, femur) are statistical averages from a given population. They are indicative, never absolute values. Anything about your own pregnancy belongs in a conversation with your gynaecologist, who knows your history and your scans.
Still waiting for a clear answer on whether you’re pregnant? You can run through our free online pregnancy test — it’s a symptom questionnaire, not a lab test, and it takes two minutes. If you’d rather play with something for a minute, try the finger pregnancy test game: it’s just a game, not a real test.
And when you have your week, these two pages are the ones I use most: what happens in each week of pregnancy and the pregnancy calculators for fertile days, due date and weeks.
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