OB Wheel: Know Your Due Date at a Glance

This free online OB wheel turns a last period, an ultrasound scan, or an IVF transfer date into a due date, gestational age, and week-by-week fetal measurements — the same digital pregnancy wheel obstetricians and midwives reach for during antenatal visits and referral notes.

Free, no sign-up 5 dating methods Works on any device

Run the OB Wheel

This is the online OB wheel itself — pick the tab that matches what you already know, fill in the fields, and press Calculate.

Estimate from Last Menstrual Period

Enter the first day of the last period and the usual cycle length to get the due date and current pregnancy age.

First day of last period

Average cycle length (days)

Estimate from an Ultrasound Scan

Works out the due date from a dated scan — most reliable when the scan falls in the first trimester.

Date of scan

Scan age — weeks

Scan age — extra days

Estimate from an IVF Embryo Transfer

Calculates dating for pregnancies conceived through assisted reproduction, using the transfer date and embryo stage.

Transfer date

Embryo stage

Find Pregnancy Age on a Specific Date

Useful for notes, referral letters, or checking what the gestational age was (or will be) on a chosen date.

First day of last period

Date to check

Cycle length (days)

Work Backward from a Due Date

When only the due date is known — for example from an old referral note — this finds the likely last period and conception date.

Due date

Cycle length (days)

What is an OB wheel?

An OB wheel — short for obstetric wheel, and also called a pregnancy wheel or gestation wheel — is the circular slide-rule clinicians have long used at the bedside to work out a due date and gestational age from the last menstrual period. It applies Naegele’s Rule: due date = LMP + 280 days, adjusted for cycle length.

This page is a digital OB wheel that does the same calculation instantly, and goes further than the physical version by pulling in scan dating, IVF transfer dating, and the expected fetal measurements for whatever gestational age it works out.

When should a due date be revised after a scan?

Ultrasound dating in early pregnancy is generally more reliable than menstrual dating. When the scan and the menstrual-based estimate disagree by more than the amounts below for that stage of pregnancy, guidance from ACOG supports moving the due date to match the scan.

Gestational Age	Best Dating Method	Difference That Triggers Redating
Up to 8w6d	Crown-rump length	More than 5 days
9w0d – 13w6d	Crown-rump length	More than 7 days
14w0d – 15w6d	BPD / HC / AC / FL	More than 7 days
16w0d – 21w6d	BPD / HC / AC / FL	More than 10 days
22w0d – 27w6d	BPD / HC / AC / FL	More than 14 days
28w0d and beyond	BPD / HC / AC / FL	More than 21 days

Pregnancy week-by-week overview

Use this alongside the calculator above to see what each stage of the pregnancy usually involves.

Week	Trimester	What's Typically Happening	Approx. Weight
4–6	1st	Implantation and rising hCG; a gestational sac becomes visible on scan	—
7–8	1st	Fetal heartbeat usually visible; ideal window for a dating scan	~1–2 g
11–13	1st	Nuchal translucency scan and combined first-trimester screening	~20–40 g
16–18	2nd	Movements often first felt; quadruple-marker screening window	~150–200 g
18–20	2nd	Detailed anomaly scan checking fetal anatomy	~200–300 g
24–28	2nd	Gestational diabetes screening; anti-D given if Rh-negative	~600–1000 g
32–34	3rd	Growth scan; Group B strep swab often considered	~1700–2150 g
36–37	3rd	Late-preterm; cervix and fetal position reviewed	~2600–2900 g
38–40	3rd	Full term; due date typically falls at 40 weeks	~3100–3500 g
41–42	3rd	Post-term; closer fetal monitoring, induction usually discussed	~3400–3700 g

Typical fetal measurements by week

These population-average figures follow the widely used Hadlock growth nomograms. The calculator above shows the full interpolated set for whatever gestational age it computes — this table is a quick-glance summary at commonly checked weeks.

Week	CRL	BPD	HC	AC	FL	Approx. Weight
12w	53mm	21mm	74mm	60mm	8mm	~23g
16w	—	32mm	124mm	99mm	21mm	~135g
20w	—	46mm	177mm	149mm	33mm	~320g
24w	—	59mm	224mm	196mm	44mm	~630g
28w	—	71mm	266mm	240mm	54mm	~1100g
32w	—	81mm	301mm	280mm	63mm	~1700g
36w	—	89mm	328mm	317mm	71mm	~2600g
40w	—	95mm	346mm	351mm	77mm	~3400g

Individual babies normally vary; always assess growth against your ultrasound machine’s own validated curve, not a table alone.

Routine antenatal testing by trimester

First visit panel: full blood count, blood group and Rh typing, antibody screen, urinalysis, rubella immunity, HIV and syphilis screening, hepatitis B/C testing, cervical cytology where due, and an early dating scan.

First trimester: booking bloods, dating scan, and combined screening around 11–13 weeks; folic acid supplementation should already be underway.

Second trimester: detailed anomaly scan around 18–20 weeks, quadruple-marker screening if the earlier window was missed, glucose tolerance testing at 24–28 weeks, and anti-D for Rh-negative mothers.

Third trimester: growth scan if clinically indicated, Group B strep swab around 35–37 weeks, cervix assessment near term, and regular fetal movement counting.

Warning signs that need prompt review

Seek prompt review for: bleeding or fluid leaking from the vagina, severe or constant abdominal pain, a noticeable drop in fetal movements, a severe headache with visual changes, sudden swelling of the face or hands, fever, persistent vomiting, or contractions before 37 weeks. Don't wait for the next scheduled appointment.

Quantitative β-hCG reference ranges

hCG is usually first detectable a little under two weeks after ovulation. In a normally progressing early pregnancy it typically rises by roughly two-thirds every two days over the first six weeks; a much slower rise can be a sign of a non-viable or ectopic pregnancy and should be followed up clinically.

Stage	Typical β-hCG Range (mIU/mL)
0–2 weeks since LMP	0 – 250
2–4 weeks	100 – 5,000
1–2 months	4,000 – 200,000
2–3 months	8,000 – 100,000
Second trimester	4,000 – 75,000
Third trimester	1,000 – 50,000

Quick glossary

Pregnancy wheel / OB wheel: a circular slide-rule traditionally used to work out due dates and gestational age by hand; this page is a digital version of the same tool.

EDD: Estimated Due Date — the projected delivery date, generally taken as 280 days after the last period.

LMP: Last Menstrual Period — the first day of the most recent period before conception.

GA: Gestational Age — how far along the pregnancy is, counted in weeks and days from the LMP.

CRL: Crown-Rump Length — the measurement used to date a pregnancy most accurately in the first trimester.

EFW: Estimated Fetal Weight, derived from ultrasound measurements.

ways to date a pregnancy — LMP, scan, IVF, a chosen date, or reverse from EDD

280

days is the standard pregnancy length used by Naegele's Rule, adjusted for cycle length

sign-ups, downloads, or fees — the calculator runs entirely in your browser

Frequently asked questions

Is this the same as an OB wheel?

Yes — OB wheel, obstetric wheel, and pregnancy wheel all refer to the same tool. This page is a digital OB wheel: it runs the same Naegele’s Rule calculation a physical wheel does, plus scan and IVF dating and a fetal measurement lookup.

What exactly does this calculator do?

It estimates your due date and current gestational age using the date of your last period, an ultrasound scan, an IVF transfer date, or a known due date — whichever information you have available — and shows the expected fetal measurements for that stage.

How accurate is dating from the last period compared with a scan?

Dating from the last period assumes a regular cycle and a fixed time to ovulation, so it can be off by several days to a couple of weeks if cycles are irregular. A first-trimester ultrasound measurement is generally considered the more reliable method when the two disagree.

Why does the calculator ask for cycle length?

A shorter or longer cycle shifts ovulation earlier or later relative to the last period, which shifts the due date by roughly the same number of days. Entering your usual cycle length keeps the estimate closer to your actual conception date.

Can I use this after an IVF transfer?

Yes. Because the embryo’s age at transfer is already known precisely (day 3 or day 5), dating from an IVF transfer tends to be very accurate — enter the transfer date and embryo stage on the IVF tab.

Does this replace a doctor's assessment?

No. It’s meant as a quick reference alongside clinical judgement, physical examination, and your own antenatal records — not as a substitute for personalised medical advice.

References

American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology. 2017.
Hadlock FP, Harrist RB, Deter RL, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984.
Federation of Obstetric and Gynaecological Societies of India. Clinical Practice Guidelines on Antenatal Care. 2021.
World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience. Geneva, 2016.

For healthcare professionals and general reference only. This calculator offers decision support and is not a substitute for clinical examination, professional judgement, or current national obstetric guidelines. Always confirm important dates and findings with your care provider.