During antenatal visits, the doctor or midwife often runs a tape measure over your belly. That simple check is called fundal height, and McDonald’s rule is the method used to turn that number into an estimate of how far along the pregnancy is. This guide explains what it is, how it is measured, how to use the formula, and where it falls short.
What Is McDonald’s Rule?
Fundal height, also called McDonald’s rule, is a measurement of uterine size that is used to assess fetal growth and development. It is taken from the top of the uterus (the fundus) down to the top of the pubic symphysis, the joint at the front of the pelvis. Wikipedia
The basic idea is simple. Between roughly 16 and 36 weeks, the measurement in centimeters is close to the gestational age in weeks for a baby lying head down. So at 24 weeks, you would expect a reading of about 24 cm. Wikipedia
How Is Fundal Height Measured?
- The woman empties her bladder first. A full bladder can push the uterus upward and change the reading by up to 3 cm. AllNurses
- She lies on her back with the knees slightly bent to relax the abdominal muscles.
- The provider finds the top of the uterus by gentle palpation.
- Using a non-stretch tape, the provider measures from the upper edge of the pubic bone to the top of the fundus.
- The value is recorded in centimeters and compared with the week of pregnancy.
In clinical practice, the actual measurement is usually recorded from around 20 weeks, and most caregivers note it at every antenatal visit. Wikipedia
Fundal Height Chart by Week
The uterus reaches certain landmarks as pregnancy advances. The chart below follows commonly used landmarks. Wikipedia
| Gestational age | Where the fundus is felt |
|---|---|
| 12 to 14 weeks | At the pubic symphysis |
| 16 weeks | Halfway between the pubic bone and the umbilicus |
| 20 to 22 weeks | At the umbilicus (belly button) |
| 36 weeks | At the xiphoid process (lower end of the breastbone) |
| 37 to 40 weeks | Slightly lower, as the baby’s head settles into the pelvis |
McDonald’s Rule Formula
Textbooks give the rule in more than one form:
- Simple form: Fundal height (cm) ≈ gestational age (weeks), for about 20 to 36 weeks. This is the version used in most antenatal clinics.
- Classic textbook form: Gestational age (weeks) = fundal height (cm) × 8 ÷ 7. Gestational age (lunar months) = fundal height (cm) × 2 ÷ 7.
Worked example (classic form): A fundal height of 28 cm gives 28 × 8 ÷ 7 = 32 weeks.
The classic form is measured over the curve of the abdomen, and the simple form uses a straight tape, which is why the two give different numbers. Exam students should follow the method taught in their own syllabus.
What Do Abnormal Readings Mean?
A difference of about 2 cm either way is usually considered normal. Some guidelines suggest suspecting poor growth when the measurement is more than 3 cm below the gestational age in weeks. clinicaltrials
Smaller than expected can be due to:
- Slow fetal growth (a small baby)
- Low amniotic fluid
- Incorrect due date
- The baby descending into the pelvis, which normally happens two to four weeks before delivery Wikipedia
Larger than expected can be due to:
- A larger baby
- Extra amniotic fluid
- Twins or multiple pregnancy
- Fibroids or an incorrect due date
In every case, an unusual reading is a reason for an ultrasound, not a diagnosis by itself.
Limitations of McDonald’s Rule
- Not very precise for dating. In a pooled analysis of five studies, 71% of pregnancies dated by fundal height were within 14 days of the ultrasound date. PubMed Central
- Technique varies. A systematic review found 19 different measurement techniques and 12 ways of converting fundal height to gestational age. PubMed Central
- Many things affect the reading: an oblique or sideways baby, Braxton Hicks contractions, a full bladder, and fetal movements. PubMed Central
- Body type matters. Landmark positions vary from woman to woman, so the tape reading is more reliable than finger-width estimates.
Where ultrasound is available, it is more accurate for dating. In many low- and middle-income countries, however, fundal height is often the only available method of estimating gestational age. PubMed Central
McDonald’s Rule vs. Johnson’s Rule
Students often confuse the two. McDonald’s rule estimates gestational age from fundal height, while Johnson’s rule estimates fetal weight using fundal height and fetal station. They are taught together but answer different questions. AllNurses
McDonald’s Rule and Your Due Date
Fundal height is a check on growth and dates, not a replacement for a proper due date calculation. To see how the estimated due date is worked out, read our guides on [EDD calculation with Naegele’s Rule] and the [Mittendorf-Williams Rule]. Near term, the [Bishop score] shows how ready the cervix is for labor.
FAQs
Is McDonald’s rule accurate? It is a useful screening tool, but ultrasound is more accurate for dating.
From which week is fundal height measured? Usually from about 20 weeks, at each antenatal visit.
Is a low fundal height dangerous? Not always. It may simply need a growth scan to check.
Can I measure it myself? A rough check is possible, but a trained provider’s measurement is more reliable.
Disclaimer: This article is for education only and is not a substitute for medical advice.
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