The halfway point of a twin pregnancy, 20 weeks gestation, is marked by the comprehensive anatomy scan, a major milestone for expectant parents. Twin pregnancies require close monitoring compared to singletons, and curiosity about the babies’ growth and development peaks at this time. The growth trajectory for twins often begins to diverge from that of a single fetus around this stage. This 20-week assessment provides specific measurements and developmental details that confirm the healthy progression of both babies.
Actual Size and Weight of 20-Week Twins
At 20 weeks, each twin measures about 25.4 to 26 centimeters (10 to 10.2 inches) in length, measured from crown to heel. They weigh approximately 300 grams (10.5 ounces) each, comparable to the size of a banana. These measurements are similar to a singleton fetus, as significant growth differences usually become more pronounced later in the third trimester.
Healthcare providers closely track inter-twin weight discordance (ITD), which is the percentage difference in estimated fetal weight (EFW) between the two fetuses. A difference of 20% to 25% or more in EFW defines growth discordance. Monitoring this difference is important because it can signal issues with the equal distribution of nutrients or placental resources.
Understanding Twin Growth Patterns
The fundamental factor influencing twin growth and the need for intense monitoring is the type of placentation, known as chorionicity. This classification dictates how resources are shared and is determined by ultrasound early in the pregnancy. Dichorionic/Diamniotic (Di/Di) twins, the most common type, each have their own separate placenta and amniotic sac, providing independent nutrient supplies.
Monochorionic twins, who are always identical, share a single placenta, which introduces risks related to resource distribution. Monochorionic/Diamniotic (Mo/Di) twins share a placenta but have separate sacs, while Monochorionic/Monoamniotic (Mo/Mo) twins share both the placenta and a single amniotic sac, representing the highest-risk scenario. The shared placenta often contains vascular connections, which can lead to unbalanced blood flow, such as in Twin-Twin Transfusion Syndrome (TTTS). Unequal sharing of the placenta is a primary cause of selective fetal growth restriction (sFGR), where one twin receives a smaller section of the shared organ.
Key Developmental Milestones at the Midpoint
At 20 weeks, the twins are reaching significant functional milestones. Their skin is producing vernix caseosa, a white, waxy coating that protects the delicate skin from constant immersion in the amniotic fluid. Fine, downy hair known as lanugo covers their bodies, helping with temperature regulation.
The digestive system is operational, with the twins swallowing amniotic fluid and producing meconium, which will form their first bowel movement after birth. Sensory development is advancing, as the brain areas responsible for taste, smell, hearing, and touch are developing rapidly. Many expectant mothers begin to consistently feel the twins’ movements, or “quickening,” a physical manifestation of their growing strength and coordination.
Monitoring Twin Growth: Measurements and Scans
The 20-week appointment centers around the detailed anatomy scan, a non-invasive ultrasound examination that assesses the health and physical structure of each twin. This scan ensures that all organs are developing as expected and that both babies are growing appropriately. Specific biometry measurements are taken to estimate the size of each fetus and track their growth progression.
These standard measurements include the Biparietal Diameter (BPD), the Head Circumference (HC), the Abdominal Circumference (AC), and the Femur Length (FL). Clinicians use these metrics to calculate the Estimated Fetal Weight (EFW) for each twin and determine the inter-twin discordance. The scan also involves checking the amniotic fluid level in each sac and assessing the umbilical cord insertion sites to identify potential risks associated with placental sharing.