Waiting to meet your baby often feels like an eternity. There are nearly ten months of waiting, guessing, analyzing about what your baby will look like and how big they will be. While modern 3-D ultrasounds can give us a peek at our little one, your doctor may perform other measurements and tests to estimate your baby’s size and make sure everything is progressing as it should.
Somewhere around the latter half of your second trimester, around 24-26 weeks of pregnancy, your doctor may begin taking a fundal height measurement. Doctors combine the fundal height with information gathered from your 20-week scan or other ultrasound tests to make predictions on how long and how much your baby will weigh.
To help you better understand fetal growth, we’ve broken down the different tests and measurements your healthcare provider may perform and what they mean.
You can also use our fetal growth chart for a detailed look at your baby’s growth and development over time.
Fundal Height
The symphysis-fundal height (SFH) is a measurement of your abdomen used to assess the baby's growth. Using a simple tape measure, the measurement is in centimeters, from the top of your pubic bone to the top of your uterus (the fundus). Typically the number of weeks you are is how many centimeters you should measure. There is an allowance of 2 cm in either direction.
For example, if you are 26 weeks pregnant, you should measure between 24 and 28 cm. Doctors can measure your fundal height between 24 and 36-37 weeks. Once your baby drops into the pelvis in late pregnancy, the measurement may not reflect their actual size.
If there is a variation of 3 cm or more, your doctor will arrange for an ultrasound scan to check your baby's growth and the amount of amniotic fluid. If the scan indicates a problem, the doctor will arrange for scans every two weeks to analyze growth patterns over time.
For example, in some instances, if a woman is obese, in multiple pregnancies (twins, triplets), and where there are large uterine fibroids, the only accurate way to measure growth is by ultrasound.
Ultrasounds
An ultrasound is a procedure that uses sound waves to project images onto a computer. They are a fairly standard procedure during prenatal check-ups, and most pregnant women will have at least two, one in her first trimester and one in the second for the 20-week scan.
The 20-week scan is a comprehensive ultrasound that checks for any abnormalities and measures your baby. This is often when most parents discover the gender of their child.
Measurements include the width of the head (biparietal diameter), the circumference of the head, and abdominal circumference, which are plotted on a graph over a period of time. The length of the upper leg bone (femur length) may also be measured.
If your baby is smaller than the 10th percentile or has a small abdomen, you may have more tests to assess their well-being. A measurement above the 90th percentile can be a sign of gestational diabetes and will need investigating. Additionally, if your baby is measuring larger than average, known as fetal macrosomia, your doctor may recommend you have a cesarean delivery or induce before your due date.
Fetal Growth Chart
If there are concerns about your baby’s size and that are not measuring the proper size for their weeks of gestation, your doctor may begin using a fetal growth chart. A fetal growth chart plots your baby’s estimated length and weight. Your doctor can then use this to look for patterns of development.
Additional Well-Being Tests
If your baby is not measuring their gestational age or you have other risk factors that may affect your baby’s development, your OB may order additional testing such as a non-stress test, stress test, biophysical profile testing, or a doppler scan.
Non-Stress Test
A non-stress test measures your baby’s heartbeat and is called a non-stress test (NST) because it does not cause any distress to the baby.
The test usually takes 40-60 minutes, during which the technician will place a heart monitor on your abdomen to assess the baby’s heartbeat.
Reasons your OB/GYN may order a non-stress test:
Diabetes
Preeclampsia or gestational hypertension
Your baby’s fetal weight seems low
Your baby is less active than normal
There is too much amniotic fluid (polyhydramnios) or too little amniotic fluid (oligohydramnios)
You’ve had a previous miscarriage
You are past your due date
Your doctor needed to turn your breech baby or you had a third-trimester amniocentesis.
A Biophysical Profile (BPP)
If, after the non-stress test, your doctor has concerns about your baby, a biophysical profile (BPP) may be done. A BPP combines the results of a CTG reading with a scan to evaluate four factors: the volume of amniotic fluid, fetal movement, fetal muscle tone and posture, and fetal breathing. Two points are given for each part of the test, so eight points would be a reassuring BPP result.
A biophysical profile may be ordered if:
Decreased physical movement
Previous stillbirth
You’re pregnant with multiples
Fetal growth restriction (FGR)
Preeclampsia or gestational hypertension
You suffer from an ongoing health condition such as lupus, renal disease, hypertension, or are prone to blood clots
Doppler Scan During Pregnancy
A doppler scan is done to measure your baby’s heartbeat and blood flow to the fetus. The sonographer places an external transducer on your abdomen to listen for accelerations in your baby's heart rate.
A doppler scan may be ordered if:
Your baby measures small (intrauterine growth restriction or IUGR).
Low birth weight is a concern
Your baby has been diagnosed with a health concern in utero
For more information on a baby’s growth and size while in the womb, consult our Fetal Growth Chart.
