Having an estimate of your baby's prenatal weight can be useful, as this information can provide knowledge about possible birth defects, developmental problems or future delivery complications. Your doctor may base the method or timing of delivery on your baby's estimated weight. However, Plus Size Pregnancy reports that in many cases, ultrasound fetal weight estimates are no more accurate than those provided by a doctor. The calculations required to estimate your baby's weight will most likely by done by a doctor or ultrasound technician. In most cases, estimated weights that are considered low or high during pregnancy are not cause for concern.
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Measure the baby's head. A measurement called the biparietal diameter assesses the diameter between the two sides of the head. This is generally done in an ultrasound no sooner than 13 weeks. According to HelpingTeens.org, the biparietal diameter usually grows from approximately 2.4 cm in the initial measurement to 9.5 cm at birth. HelpingTeens.org notes that because babies of similar weights may have different head sizes, this data is not always reliable later in pregnancy.
Measure the length of the baby's femur. The femur is the body's longest bone, and is considered an indicator of your baby's growth in terms of length. According to HelpingTeens.org, this number will usually increase from about 1.5 cm to about 7.8 cm at birth.
Measure the circumference of the abdomen. The circumference of the baby's abdomen is an important ultrasound measurement for assessing your baby's weight late in your pregnancy.
Use polynomial equations featuring the biparietal diameter, femur length and abdominal circumference to arrive at the estimated fetal weight. This can be done at any stage of pregnancy, notes HelpingTeens.org. This information will likely be calculated by your health care provider. Various types of computer software and charts can be used for this calculation (see Resources).