During a surrogacy journey, ultrasound reports are often the only window intended parents have into their baby’s development. We understand how unsettling it can feel—especially when you are far away—to see phrases like “<5th percentile” or “estimated fetal weight: 10 lbs” appear on a report.
In clinical reality, however, many of these findings are far less alarming than they appear. Both medical evidence and extensive real-world experience show that most ultrasound “red flags” reflect the inherent limitations of prenatal imaging, not true fetal disease.
This article explains why ultrasound measurements—particularly fetal weight and limb length—are often inaccurate, how common false alarms really are, and how intended parents can interpret these results with clarity and confidence.
Key Takeaways
- Ultrasound is not a scale. It estimates fetal weight using mathematical formulas rather than direct measurement. In the third trimester, a single ultrasound commonly has an error margin of ±10–15% or more, as acknowledged by American College of Obstetricians and Gynecologists.
- “Big baby” is frequently a false alarm. When ultrasound suggests fetal macrosomia, studies summarized by ACOG show the positive predictive value is only about 30%–50%.
- Genetic screening matters more than isolated measurements. If PGT/PGS or prenatal NIPT results are low risk, an isolated finding such as short femur length rarely indicates chromosomal abnormality.
- Growth charts are not one-size-fits-all. Common ultrasound standards were developed decades ago using predominantly Caucasian populations and may misclassify fetuses from Asian or smaller-framed families.
1. Why Does Ultrasound Weight Estimation Vary So Much?
Many intended parents ask the same question: “The ultrasound said 10 pounds—why was the baby born weighing only 8?”
This discrepancy is not guesswork. It is a predictable consequence of how ultrasound works.
Ultrasound solves a geometry problem, not a weighing problem
Fetal weight is calculated using biometric measurements—head circumference, abdominal circumference, and femur length—plugged into formulas such as the Hadlock equation, which was developed from U.S. populations in the 1980s and remains the most widely used standard today. Small differences in any measurement (even a few millimeters) are mathematically amplified, resulting in large swings in estimated weight.
Among these measurements, abdominal circumference (AC) carries the greatest weight in fetal weight formulas—and also the greatest variability, making it a common source of error.
Imaging and technical limitations matter
In late pregnancy, fetal position, reduced amniotic fluid, increased fetal fat, and maternal abdominal tissue can blur anatomical borders. This often leads to overestimation of abdominal circumference, which disproportionately inflates weight estimates. As a result, ultrasound tends to overestimate larger babies.
Real Case Example: Two Ultrasounds, Two Very Different Predictions
In one surrogacy case, a gestational carrier underwent two ultrasounds on consecutive days:
- Day 1 (OB ultrasound): Estimated fetal weight 7 lb 12 oz
- Day 2 (MFM ultrasound): Estimated fetal weight likely over 9 lb
Concerned about delivery safety and shoulder dystocia risk, the maternal-fetal medicine (MFM) physician recommended cesarean delivery. Although the surrogate had hoped for a vaginal birth, the decision was made conservatively.
The baby was ultimately born weighing 8 lb 2 oz—between the two estimates.
This case illustrates a key reality: even within 48 hours, different physicians, equipment, and measurement techniques can produce markedly different ultrasound weight estimates. In the face of uncertainty, medical decisions often favor safety rather than precision.
Why “Big Baby” Is Often a False Alarm
Clinical data consistently show that ultrasound is far better at ruling out macrosomia than confirming it.
According to evidence summarized by ACOG:
- When ultrasound suggests fetal macrosomia, the positive predictive value is only about 30%–50%. → At least half of fetuses labeled “very large” on ultrasound are born at a normal weight.
- For estimated birth weights above 4,500 g (≈10 lbs), ultrasound sensitivity has been reported as low as 10%–45%, with a high false-positive rate.
- In contrast, when ultrasound does not suggest macrosomia, its ability to rule out a truly large baby exceeds 90–95%.
For this reason, professional guidelines caution against making delivery decisions based solely on ultrasound-estimated fetal weight.
2. “Short Femur” on Ultrasound: Disease or Normal Variation?
Seeing “Femur Length <5th percentile” on an anatomy scan understandably raises fears of skeletal or chromosomal disorders.
Modern maternal-fetal medicine emphasizes context over isolated numbers.
Guidance from the Society for Maternal-Fetal Medicine indicates that when prenatal genetic screening—such as PGT/PGS or noninvasive prenatal testing (NIPT)—is low risk, an isolated short femur length, in the absence of other structural abnormalities, does not typically warrant invasive diagnostic testing.
In many cases, especially among Asian families, this finding represents what MFM specialists describe as “constitutionally small”—a normal genetic growth pattern rather than a medical disorder.
From a technical standpoint, femur length can also appear shorter if the leg is not perfectly aligned with the ultrasound beam. In simple terms, sometimes the baby just isn’t in the ideal position—or “pose”—for measurement.
Real Case Example: When a Second Opinion Changes Everything
In one anatomy scan, fetal limb measurements were reported below the 5th percentile using standard Hadlock curves, causing significant anxiety.
A detailed follow-up ultrasound was later performed by an experienced MFM physician. After a comprehensive structural evaluation, the conclusion was reassuring: the fetus was completely healthy.
The explanation was straightforward:
- Hadlock growth curves were developed primarily from Caucasian populations
- Asian fetuses often have different limb-to-body proportions
- The intended mother’s height was approximately 155 cm (5’1″), strongly influencing genetic growth patterns
The pregnancy continued uneventfully, and the baby was born healthy. Years later, the child was thriving—active, developmentally normal, and full of life.
This case highlights a crucial principle: percentiles describe population statistics, not individual destiny.
3. Does the Gestational Carrier Cause These Variations?
Some intended parents worry whether ultrasound findings reflect something the surrogate did—or failed to do.
While the uterine environment does influence fetal development to a degree, skeletal size and proportions are overwhelmingly determined by genetics, not diet or lifestyle. As long as the gestational carrier has passed medical screening and receives appropriate prenatal care, most ultrasound fluctuations represent measurement noise rather than caregiving issues.
Frequently Asked Questions
1. Should we induce labor if ultrasound estimates the baby at 10 pounds?
In most cases, no. ACOG guidelines recommend against induction or cesarean delivery solely for suspected macrosomia unless estimated weight exceeds approximately 5,000 g (11 lb) in non-diabetic pregnancies.
2. Do we need amniocentesis for short femur findings?
Usually no. If PGT/PGS or NIPT results are normal, additional invasive testing is typically not recommended for isolated short femur length.
3. Why do percentiles change from scan to scan?
This is normal. Fetal growth is not linear, and small measurement differences are common. Trends over time matter far more than any single value.
4. Does maternal BMI affect ultrasound accuracy?
Yes, it can. Increased abdominal tissue can reduce image clarity and increase measurement variability—often leading to overestimation of fetal size.
5. Do “small” babies on ultrasound have poorer outcomes later?
Most do not. Many infants labeled as small prenatally demonstrate normal growth and development after birth.
Final Thoughts
Statistics are abstract. Babies are not.
Ultrasound reports provide valuable information, but they are approximations—not predictions. In surrogacy, where emotional investment is profound and waiting is long, understanding the limitations of prenatal imaging can make the journey far less stressful.
If you ever feel uncertain about how to interpret an ultrasound report or medical recommendation, experienced professional guidance can make all the difference.
Our team is here to help you navigate both the science and the emotions of the surrogacy journey—with clarity, compassion, and experience.
References & Clinical Sources
- American College of Obstetricians and Gynecologists (ACOG). Practice Bulletin No. 216: Macrosomia.
- Society for Maternal-Fetal Medicine (SMFM). Consult Series on fetal growth abnormalities.
- Hadlock FP et al. Estimation of fetal weight with the use of head, body, and femur measurements.
- American Journal of Obstetrics & Gynecology, Radiology — peer-reviewed studies on third-trimester ultrasound accuracy.
