Table of Contents
Introduction:
The accurate estimation of fetal weight is very important, as it is closely associated with a fetus's well-being and survival. Ultrasound-based fetal weight estimation is the most widely practiced technique. So there are different ultrasound-based models developed for fetal weight estimation. The differences in genetic, nutritional, and socioeconomic factors can impact fetal growth, and no single ultrasound-based model is equally applicable to all populations. The fetal weight estimation is essential to eliminate the potential complications that can arise from excessive or low fetal birth weight during delivery.
What Is the Importance of Estimating Fetal Weight?
Both low and excessive fetal weight at delivery is linked with an increased risk of newborn complications during labor. Complications associated with different birth weights are as follows:
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The complications associated with low birth weight are fetal prematurity. It can also arise due to intrauterine growth restriction.
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The complications associated with large babies can be shoulder dystocia (when baby’s shoulders get stuck inside the pelvis during birth), bony injuries, brachial plexus injuries, metabolic abnormalities (such as low calcium and blood sugar), higher hemoglobin levels, various congenital abnormalities, jaundice (yellowish discoloration of the skin), intrapartum asphyxia.
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Almost one-third of large babies have feeding difficulties during an initial period.
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Mothers also get injuries if the baby is large; the potential damage for the mothers can include birth canal injuries, postpartum hemorrhage, and pelvic floor injury damage. So by estimating the fetal weight, proper management of the complication can be planned ahead of time.
If the estimated weight suggests that a baby is big for its gestation period, then the doctors plan a scheduled cesarean or an induction. If a baby is estimated to weigh ten pounds or more, it is known as macrosomia, and this condition needs a C-section. The primary concern with the vaginal delivery of a big baby is shoulder dystocia. This can happen when the baby gets stuck in the birth canal and which can lead to damage to the shoulder nerve and arm. Shoulder dystocia can lead to permanent paralysis of the baby's arm muscles and injury to the mother.
What Factors Can Affect the Birth Weight of the Babies?
The birth weight of the babies can be influenced by several factors that are:
Large Babies: A fetus can be born large when the mother is overweight or the parents are large. The following complications can occur when the baby is big:
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If the pregnancy lasts longer than forty-two weeks.
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Abnormalities in fetal chromosomes.
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If the mother gets diabetes before or during pregnancy.
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Mothers gain a substantial amount of weight during pregnancy.
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If the growth of the fetus is overstimulated in the uterus.
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The mother's ethnicity.
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Having a boy.
Small Babies: A baby may be born small due to the following reasons:
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Preterm birth (born before the estimated due date).
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Born to too small parents.
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The mother's ethnicity.
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Abnormalities in fetal chromosomes.
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Chronic diseases in mothers such as heart, kidney disease, or high blood pressure.
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Malnutrition.
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Substance abuse during pregnancy.
When Is the Fetal Weight Calculated?
The baby's weight is calculated along the ultrasound done during the 20 to 22 weeks of pregnancy, and this scan is also called morphology ultrasound. This is performed by a sonographer. This milestone scan is strongly recommended as this checks for both a detailed assessment of any abnormalities and the baby's weight. After birth, the babies can be weighed using baby weight scales that are available online.
How Is the Weight of the Baby Measured Through Ultrasound?
During the ultrasounds, the sonographer or a specialist will determine the baby's weight in the mother's uterus. The specialist can use many types of parameters to estimate the baby's weight. Head circumference (HC), biparietal diameter (BPD), femur length (FL), and abdominal circumference (AC) are the most commonly used parameters. The specialist uses the combination of the measurements mentioned above in a formula to calculate the fetus's weight and size.
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Biparietal Diameter - This is the distance between the sides of the fetus's head. The biparietal diameter can be different for different babies that weigh the same. So, this measurement is not always reliable at the later stage of pregnancy.
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Head Circumference - This measures the circumference of the fetus's head.
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Abdominal Circumference - This measures the circumference of the fetus's abdomen. This measurement is achieved by measuring the distance between the top of the mother's belly and pubic bone with a measuring tape.
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Femur Length - This is measured by the length of the longest bone (upper thigh bone). It can differ in babies of different weights, so this measurement collected cannot always be reliable.
What Is Fundus Height?
The symphysis-fundal height (SFH) is a measurement of the mother's abdomen used to assess the baby's growth. Using a simple tape, the measurement from the top of the pubic bone to the top of the uterus is taken. The measurements are taken in centimeters. Normally, the measurement of the fundus length should be the number of weeks of pregnancy. There is an allowance of two centimeters in either direction. For example, if a woman is 26 weeks pregnant, then the fundus length should measure between 24 and 28 cm. The health care professionals can measure the fundal height between 24 and 36 to 37 weeks.
If there is a variation of three centimeters or more, the doctor will perform an ultrasound scan to check the amount of amniotic fluid and the baby's growth. If there is any problem detected in the scan, then the scan will be done every two weeks to analyze growth patterns over time. If a woman is obese, if there are large uterine fibroids, and in multiple pregnancies (twins, triplets), the only accurate way to measure the growth of the fetus is by ultrasound.
How Accurate Is Ultrasound Measurement?
There are many different formulae for estimating fetal weight. It is essential to know there is a recognized 15 percent error in fetal weight estimation through ultrasound. This means the baby can be either 15 percent larger or 15 percent smaller than the estimated weight provided by the ultrasound. Ultrasound scans are usually more accurate for assessing the size of the baby during the first half of pregnancy. But as babies develop, they can grow faster.
Conclusion:
Estimation of the fetal weight is done during the 20 to 22 weeks of pregnancy and even at the end of the third trimester. This helps the health care professionals to know the size and weight of the baby. The ultrasound measurements are mostly accurate. This helps the doctors to plan the treatment accordingly.
