Monitoring fetal growth is particularly important in twin pregnancies. Ultrasound is the primary tool used to track development and assess fetal well-being based on gestational age. Regular assessments are necessary to identify deviations from the expected growth curve, which can signal potential complications unique to twins.
The Expected Weight Range
At 24 weeks of gestation, a typical twin fetus is expected to have an Estimated Fetal Weight (EFW) that falls within a specific range. For dichorionic twins, who have separate placentas, the median weight is approximately 456 grams, or about one pound. The normal range for these twins, spanning the 10th to the 90th percentile, is roughly 420 to 520 grams, or 14.8 to 18.3 ounces.
Monochorionic twins, who share a single placenta, tend to be slightly smaller. Their median weight at 24 weeks is closer to 430 grams, with the normal range clustering just below that of dichorionic twins. Staying within the 10th and 90th percentiles is considered an indication of healthy growth. All twins are typically smaller than singleton fetuses at this stage, and their growth trajectories diverge further in the third trimester.
The Significance of the 24-Week Milestone
The 24-week mark is a significant benchmark because it is widely considered the threshold of fetal viability. If birth were to occur at this time, the twins would have a reasonable chance of survival with intensive medical care. This potential is largely due to the development of the babies’ lungs, which begin to produce surfactant. Surfactant is a lipoprotein that coats the inner surface of the lungs, preventing air sacs from collapsing and enabling the baby to breathe effectively.
Achieving this milestone signals a change in clinical management, often leading to intensified monitoring protocols. The focus shifts to maintaining the pregnancy while preparing for the possibility of an extremely preterm birth. Medical teams specializing in high-risk obstetrics and neonatology are brought into consultation, and intervention plans are established. Survival rates for twins born at 24 weeks are approximately 60 to 70%, depending heavily on the quality of neonatal intensive care available.
Factors Influencing Twin Growth
Twin growth can deviate from the expected range due to several variables, the most significant of which is chorionicity, the number of placentas present. Dichorionic twins, who have two separate placentas, generally experience growth patterns similar to singletons until the third trimester. Monochorionic twins, who share a single placenta, face a higher risk of complications because of the vascular connections between the two circulations within that shared organ.
This shared placental architecture is the root cause of two major growth complications: Selective Fetal Growth Restriction (sFGR) and Twin-to-Twin Transfusion Syndrome (TTTS). In sFGR, one twin receives a disproportionately small share of the placenta, resulting in an Estimated Fetal Weight (EFW) below the 10th percentile and an inter-twin weight difference of 25% or more. This disparity in placental territory limits the nutrient and oxygen supply to the smaller twin, leading to restricted growth.
TTTS is a complication where blood is unevenly exchanged between the twins through shared vascular connections. This leads to a chronic volume imbalance: the donor twin is often smaller, anemic, and has low amniotic fluid. Conversely, the recipient twin is larger, polycythemic, and has excess amniotic fluid. Both sFGR and TTTS require specialized, frequent monitoring and may necessitate early intervention.
Monitoring Growth and Interpreting Scans
The weight of each twin is not measured directly but is estimated using the Estimated Fetal Weight (EFW) calculation. EFW is derived from specific measurements taken during a detailed ultrasound scan. Key measurements include the biparietal diameter (BPD) and head circumference (HC) for head size, and the abdominal circumference (AC) and femur length (FL) for body mass and long bone growth.
These measurements are then plugged into established formulas, such as the Hadlock equations, to calculate the EFW. Medical providers do not compare these EFWs to singleton growth charts; instead, they use customized growth curves specific to twins to accurately assess the growth pattern. These twin-specific charts account for the naturally lower weight trajectory of multiples.
Beyond size, Doppler flow studies assess blood flow through the umbilical cord and other fetal vessels, providing insight into placental function. Abnormal Doppler flow, especially in the umbilical artery, can indicate placental insufficiency or growth restriction before the EFW falls significantly. If the EFW is consistently below the 10th percentile or if significant weight discordance is noted, surveillance may increase to weekly visits, and preparations for early delivery may begin.