Fundal height, or McDonald's rule, is a measure of the size of the uterus

Uterus

The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

 used to assess fetal growth

Gestational age

Gestational age relates to the age of an embryo or fetus . There is some ambiguity in how it is defined:*In embryology, gestational age is the time elapsed since conception. This interval is also termed fertilisation age....

 and development. It is measured from the top of the mother's uterus to the top of the mother's pubic bone in centimeters. It should match the fetus

Fetus

A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

' gestational age in weeks within 1 to 3 cm, e.g., a pregnant woman's uterus at 26 weeks should measure 23 to 29 cm. This is valid from 24 weeks.

Most caregivers will record their patient's fundal height on every prenatal visit. Measuring the fundal height can be an indicator of proper fetal growth and amniotic fluid development.

Knowledge of gestational age may impact how the height is measured.

Shorter measure

A shorter measure can happen for one of the following reasons:

• Fetus descent into the pelvis, seen normally two to four weeks before delivery
• Error in estimated date of pregnancy based on first day of last menstrual period
• Fetus is healthy but physically small
• Oligohydramnios
  Oligohydramnios
  Oligohydramnios is a condition in pregnancy characterized by a deficiency of amniotic fluid. It is the opposite of polyhydramnios.-Diagnosis:Diagnosis is made by ultrasound measurement of the amniotic fluid index...
  
  
• Fetus positioned sideways
• Small for gestational age
  Small for gestational age
  Small for gestational age babies are those who are smaller in size than normal for the baby's sex and gestational age, most commonly defined as a weight below the 10th percentile for the gestational age.-Terminology:...
  
  

Longer measure

On the other side, a longer measure can be caused by:

• Twins
  TWINS
  Two Wide-Angle Imaging Neutral-Atom Spectrometers are a pair of NASA instruments aboard two United States National Reconnaissance Office satellites in Molniya orbits. TWINS was designed to provide stereo images of the Earth's ring current. The first instrument, TWINS-1, was launched aboard USA-184...
  
  , or other types of multiple birth
  Multiple birth
  A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple births are used, depending on the number of offspring. Common multiples are two and three, known as twins and triplets...
  
  
• Error in estimated date of conception
• Fetus is healthy but physically large
• Gestational diabetes
  Gestational diabetes
  Gestational diabetes is a condition in which women without previously diagnosed diabetes exhibit high blood glucose levels during pregnancy . It is widely accepted as a disease only in the United States, there is some question whether the condition is natural during pregnancy...
  
   causing a larger baby
• Polyhydramnios
  Polyhydramnios
  Polyhydramnios is a medical condition describing an excess of amniotic fluid in the amniotic sac. It is seen in 0.2 to 1.6% of pregnancies,,...
  
  
• Large for gestational age
  Large for gestational age
  Large for gestational age is an indication of high prenatal growth rate, often defined as a weight that lies above the 90th percentile for that gestational age...
  
  
• Hydatidiform Mole
  Hydatidiform mole
  Molar pregnancy is an abnormal form of pregnancy, wherein a non-viable, fertilized egg implants in the uterus, and thereby converts normal pregnancy processes into pathological ones. It is characterized by the presence of a hydatidiform mole...
  
  
• Breech birth
  Breech birth
  A breech birth is the birth of a baby from a breech presentation. In the breech presentation the baby enters the birth canal with the buttocks or feet first as opposed to the normal head first presentation....
  
  

As a pregnancy

Pregnancy

Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

approaches its end, the fundal height will become less accurate.