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Estimate your baby's current weight using gestational age or ultrasound measurements
| Week | Weight (grams) | Weight (pounds) | Length (inches) |
|---|---|---|---|
| 8 weeks | 1g | 0.00 lbs | 0.63 in |
| 12 weeks | 14g | 0.03 lbs | 2.13 in |
| 16 weeks | 100g | 0.22 lbs | 4.57 in |
| 20 weeks | 300g | 0.66 lbs | 6.46 in |
| 24 weeks | 600g | 1.32 lbs | 11.81 in |
| 28 weeks | 1000g | 2.20 lbs | 14.8 in |
| 32 weeks | 1700g | 3.75 lbs | 16.69 in |
| 36 weeks | 2600g | 5.73 lbs | 18.66 in |
| 37 weeks | 2900g | 6.39 lbs | 19.13 in |
| 38 weeks | 3100g | 6.83 lbs | 19.61 in |
| 39 weeks | 3300g | 7.28 lbs | 20 in |
| 40 weeks | 3400g | 7.50 lbs | 20.16 in |
A fetal weight calculator estimates your baby's current weight during pregnancy using either gestational age or ultrasound measurements. This tool is important for monitoring fetal growth and development, helping healthcare providers identify potential growth issues early. Estimated fetal weight (EFW) is compared against standard growth charts to determine if the baby is growing appropriately for their gestational age. Babies measuring significantly above or below average percentiles may require additional monitoring or interventions to ensure optimal outcomes.
The most accurate method uses ultrasound measurements of specific fetal dimensions combined with mathematical formulas, most commonly the Hadlock formula. During ultrasound, technicians measure the biparietal diameter (width of the head), head circumference, abdominal circumference, and femur length. These measurements are input into validated formulas that estimate fetal weight based on extensive research correlating these measurements with actual birth weights. While ultrasound-based estimates are more precise than gestational age alone, they still have a margin of error of approximately ±15%.
Understanding your baby's estimated weight helps identify conditions like intrauterine growth restriction (IUGR), where the baby is smaller than expected, or macrosomia, where the baby is larger than average. Both conditions may require special management. Small babies may need additional monitoring, earlier delivery, or investigation into causes like placental insufficiency or genetic conditions. Large babies may increase risks of delivery complications, prompting discussions about delivery timing or method. Regular fetal weight monitoring throughout pregnancy provides valuable information about your baby's health and development trajectory.
The simplest method estimates fetal weight based on gestational age using average growth curves. Standard fetal growth charts provide expected weights for each week of pregnancy. For example, at 30 weeks, the average fetal weight is approximately 1,300-1,400 grams (about 3 pounds). However, this method provides only a rough estimate because individual babies grow at different rates depending on genetic factors, maternal nutrition, placental function, and other variables. It's useful for general understanding but lacks the precision needed for medical decision-making.
The most common clinical method uses the Hadlock formula, which incorporates four ultrasound measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The formula is: Log10(EFW) = 1.335 - 0.0034(AC)(FL) + 0.0316(BPD) + 0.0457(AC) + 0.1623(FL), where measurements are in millimeters. This formula was developed by studying thousands of pregnancies and correlating ultrasound measurements with actual birth weights. The result provides an estimated fetal weight with approximately 85% accuracy within ±15% of actual weight.
Healthcare providers assess fetal weight in context of growth percentiles. After calculating estimated weight, it's plotted on growth charts showing the distribution of fetal weights at each gestational age. Percentiles indicate where your baby falls compared to other babies at the same gestational age. The 50th percentile represents average weight, while the 10th and 90th percentiles mark the boundaries of typical variation. Babies below the 10th percentile may have IUGR, while those above the 90th percentile are considered large for gestational age (LGA).
It's important to understand that estimated fetal weight has limitations. Ultrasound accuracy depends on many factors including maternal obesity (which can obscure measurements), baby's position, amniotic fluid levels, and technician skill. The margin of error increases with larger babies and in late pregnancy. Rather than relying on a single measurement, providers track growth trends over time. A baby consistently measuring at the 30th percentile may be perfectly healthy, while a baby dropping from the 70th to 30th percentile over several weeks might indicate a growth problem requiring investigation.
Eat a balanced diet rich in protein, healthy fats, fruits, and vegetables to support optimal fetal growth.
Take prenatal vitamins daily, including adequate folic acid, iron, and DHA for brain development.
Attend all scheduled appointments for growth monitoring, ultrasounds, and early detection of potential issues.
Eliminate smoking, alcohol, and drugs, which can significantly restrict fetal growth and development.
Control diabetes, hypertension, and other conditions that can affect placental function and fetal growth.
Get adequate sleep and manage stress, as chronic stress can impact blood flow to the placenta.
Ultrasound fetal weight estimates have a margin of error of approximately ±15%, meaning the actual weight could be 15% higher or lower than estimated. Accuracy is best in the second trimester and decreases slightly in late pregnancy. Factors affecting accuracy include maternal obesity, low amniotic fluid, baby's position, and measurement technique. Despite limitations, ultrasound provides the most reliable non-invasive method for estimating fetal weight during pregnancy.
A baby measuring below the 10th percentile may have intrauterine growth restriction (IUGR). This doesn't always indicate a problem—some babies are constitutionally small due to genetics. However, it may signal placental insufficiency, maternal health issues, or fetal abnormalities. Your provider will perform additional testing including Doppler ultrasounds to assess blood flow, more frequent monitoring, and possibly early delivery if growth stagnation is detected.
Babies measuring above the 90th percentile are considered large for gestational age (LGA) or macrosomic if over 4,000-4,500 grams. Causes include gestational diabetes, maternal obesity, or genetic factors. Large babies may face delivery complications like shoulder dystocia and have higher rates of cesarean delivery. Your provider may recommend glucose testing, additional monitoring, and discussions about delivery timing and method to optimize outcomes.
Fetal weight estimates made close to delivery provide reasonable predictions of birth weight, but significant variations occur. Studies show that estimates made within a week of delivery are accurate within 10-15% about 70-85% of the time. The estimate becomes less accurate for very small or very large babies. Growth trends throughout pregnancy are more informative than single measurements for assessing fetal health and planning delivery.
For low-risk pregnancies, fetal weight is typically estimated during the anatomy scan (18-22 weeks) and sometimes in the third trimester. High-risk pregnancies may require more frequent monitoring—every 2-4 weeks if growth concerns exist. Your provider determines monitoring frequency based on risk factors including maternal age, medical conditions, previous pregnancy complications, multiple gestation, or abnormal initial measurements.
Multiple factors influence fetal weight including genetics (parental size), maternal nutrition and weight gain, placental function, maternal health conditions (diabetes, hypertension), smoking, alcohol use, multiple gestation, and gestational age. Some factors are modifiable (nutrition, substance use) while others are not (genetics, placental location). Your healthcare provider helps optimize controllable factors to support healthy fetal growth.
If growth restriction is due to maternal nutrition or lifestyle factors, improvements may help. However, placental insufficiency or genetic factors often cannot be changed. Your provider may recommend increased protein intake, bed rest, or monitoring for optimal delivery timing. In some cases, early delivery is safer than continuing pregnancy if the placenta isn't adequately supporting the baby. Follow your provider's specific recommendations.
Twins typically grow at similar rates to singletons until about 32 weeks, then growth may slow slightly as uterine space becomes limited. Twin-specific growth charts are used for comparison. Each twin is monitored individually, as growth discordance (significant size difference between twins) may indicate twin-to-twin transfusion syndrome or other complications requiring specialized management. Regular ultrasound monitoring is standard for multiple pregnancies.
This calculator provides estimates for educational purposes only and should not replace professional medical advice. Fetal weight estimates have a margin of error of ±15% and should be interpreted by qualified healthcare providers in context of your complete medical history. Only your healthcare provider can accurately assess fetal growth, interpret ultrasound measurements, and make recommendations about pregnancy management. If you have concerns about your baby's growth or development, consult your healthcare provider immediately.