Your Baby Print Can They Tell How Big the Baby Is by Ultrasound? Medically reviewed by facebook linkedin instagram Medically reviewed by Anita Sadaty, MD on July 23, 2016 Anita Sadaty, MD, is a board-certified obstetrician-gynecologist at North Shore University Hospital and founder of Redefining Health Medical. Learn about our Medical Review Board Anita Sadaty, MD Written by twitter linkedin Written by Robin Elise Weiss, PhD Robin Elise Weiss, PhD, MPH is a professor, author, childbirth and postpartum educator, certified doula, and lactation counselor. Learn about our editorial policy Robin Elise Weiss, PhD Updated on September 19, 2019 Your Pregnancy Week by Week Overview First Trimester Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8 Week 9 Week 10 Week 11 Week 12 Week 13 Second Trimester Week 14 Week 15 Week 16 Week 17 Week 18 Week 19 Week 20 Week 21 Week 22 Week 23 Week 24 Week 25 Week 26 Week 27 Third Trimester Week 28 Week 29 Week 30 Week 31 Week 32 Week 33 Week 34 Week 35 Week 36 Week 37 Week 38 Week 39 Week 40 Photo © Blend Images/Getty Images Ultrasound exams are notoriously inaccurate for predicting the weight of your baby. The ultrasound gives an estimate of the weight of your baby, but this estimate can be off a pound or more in either direction. There are multiple ways to predict the weight via ultrasound, and the results can vary widely. It is generally not advisable to make decisions about induction or a planned cesarean section based solely on estimated fetal weight. How Is Weight Predicted? There are more than 30 different algorithms used to predict the weight of your baby via ultrasound. Most use common ultrasound measurements like your baby's head circumference (via biparietal diameter), abdominal circumference, femur length, and others. BPD Measurements and Your Baby's Fetal Weight Various programs add the sex of the baby, the gestational age, and other factors into the mix. For example, babies of diabetic mothers can be larger due to maternal high blood sugar, and some estimates assume these babies will be larger. Your midwife or doctor may also try to predict the fetal weight by using their hands during a physical exam of your abdomen using Leopold's Maneuvers, which also helps determine the position of the baby. Leopold's Maneuvers is not an accurate tool for predicting the true weight of the baby, though some practitioners are better than others at making weight estimates this way. Estimate-Based Inductions or Cesareans When the estimated weight suggests the child is large for gestational age, some doctors will plan an induction or scheduled cesareans. In general, a baby who is estimated to be 10 pounds or more—known as macrosomia—may warrant a C-section. The primary concern with vaginal delivery of a large baby is shoulder dystocia. This occurs when the baby's body gets stuck in the birth canal and may cause damage to arm and shoulder nerves. This can result in permanent paralysis of the child's arm muscles as well as injury to the mother. While cesareans are sometimes performed for large babies, the babies may actually be several pounds under the pre-birth estimate. This is why many people do not recommend using this estimate to make decisions about the mode of birth. Sometimes estimates are under as well. Warning Signs of Shoulder Dystocia During Birth Third-Trimester Ultrasounds If you are being asked to do an ultrasound in the last trimester of pregnancy, ask what the ultrasound is being used to tell you. Does your practitioner have a specific concern that only an ultrasound can answer? Is there something going on? Or is it a routine procedure done on most women in the practice. Some things that your practitioner may want to look at in the third trimester include, but are not limited to: Placental locationThe position of the babyAmount of amniotic fluidBiophysical profile Be sure to talk to your doctor or midwife about their thoughts. Even if your baby is on the larger size, this does not mean that you will be unable to give birth vaginally. The size of the baby is only one piece of the puzzle. 5 Things to Ask Your Doctor Before an Induction Was this page helpful? Thanks for your feedback! Get diet and wellness tips to help your kids stay healthy and happy. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Milner J, Arezina J. The accuracy of ultrasound estimation of fetal weight in comparison to birth weight: A systematic review. Ultrasound. 2018;26(1):32-41. doi:10.1177/1742271X17732807 Ladfors L, Shaat N, Wiberg N, Katasarou A, Berntorp K, Kristensen K. Fetal overgrowth in women with type 1 and type 2 diabetes mellitus. PLoS One. 2017;12(11):e0187917. doi:10.1371/journal.pone.0187917 Wei Y, Yang H. [Variation of prevalence of macrosomia and cesarean section and its influencing factors]. Zhonghua Fu Chan Ke Za Zhi. 2015;50(3):170-6. doi:10.3760/cma.j.issn.0529-567x.2015.03.002 Hill MG, Cohen WR. Shoulder dystocia: prediction and management. Womens Health (Lond). 2016;12(2):251-261. doi:10.2217/whe.15.103 Blouin D, Rioux C. Routine third trimester control ultrasound examination for low-lying or marginal placentas diagnosed at mid-pregnancy: is this indicated? J Obstet Gynaecol Can. 2012;34(5):425-428. doi:10.1016/S1701-2163(16)35238-0 Hebbar S, Rai L, Adiga P, Guruvare S. Reference ranges of amniotic fluid index in late third trimester of pregnancy: what should the optimal interval between two ultrasound examinations be? J Pregnancy. 2015;2015:319204. doi:10.1155/2015/319204 Additional Reading Bajracharya J, Shrestha NS, Karki C. Accuracy of prediction of birth weight by fetal ultrasound. Kathmandu Univ Med J (KUMJ). 2012;10(38):74-6. PMID: 23132481 Barel O, Vaknin Z, Tovbin J, et al. Assessment of the accuracy of multiple sonographic fetal weight estimation formulas: a 10-year experience from a single center. J Ultrasound Med. 2013;32(5):815. doi:10.7863/ultra.32.5.815 Baxley EG, Gobbo RW. Shoulder dystocia. Am Fam Physician. 2004;69(7):1707-14. Blackwell SC, Refuerzo J, Chadha R, et al. Overestimation of fetal weight by ultrasound: does it influence the likelihood of cesarean delivery for labor arrest? Am J Obstet Gynecol. 2009;200:340.e1-340.e3. doi:10.1016/j.ajog.2008.12.043 Colman A, Maharaj D, Hutton J, Tuohy J. Reliability of ultrasound estimation of fetal weight in term singleton pregnancies. N Z Med J. 2006;119(1241):U2146. PMID: 16964298 Continue Reading