Expectant parents of twins often have questions about their babies’ development, especially concerning size and growth. At 16 weeks, your twins are undergoing significant changes. Understanding these milestones offers insight into their rapid development as they continue to grow.
Typical Size and Measurements
At 16 weeks gestation, each twin measures about 11.4 to 12 centimeters (approximately 4.5 to 4.7 inches) from crown to rump, roughly the size of an avocado. Each twin usually weighs around 99 to 100 grams (about 3.5 ounces).
While a singleton baby at 16 weeks might be slightly heavier, the crown-rump length of twins is comparable at this stage. As pregnancy progresses, twins often experience slower growth compared to singletons, particularly after 28 to 32 weeks, due to the shared uterine environment. At 16 weeks, their proportional growth continues, with bodies growing faster than heads.
Developmental Milestones
At 16 weeks, your twins are developing various bodily systems. Their unique fingerprints have nearly finished forming, and tiny fingernails are growing. They are becoming more active, engaging in movements such as kicking, rolling, and even performing handstands within the womb.
These movements include reflexes like grasping, sucking, swallowing, yawning, and hiccupping, which help prepare them for life outside the uterus. Internally, their circulatory systems are well-established, with hearts pumping around 25 quarts of blood daily. Their kidneys are functioning, processing and expelling amniotic fluid.
Sensory development is advancing, with ears positioned on the sides of their heads and developing hearing. Soon, they may recognize sounds, including your voice. Their eyes have moved from the sides to the front of their heads, and they can move them slowly behind closed eyelids. Fine, downy hair called lanugo covers their skin, and external genitals are typically developed enough to determine sex via ultrasound.
Factors Influencing Growth
Several factors can influence twin growth and size at 16 weeks. The type of twinning plays a role, particularly whether they are monochorionic or dichorionic. Dichorionic twins each have their own placenta and amniotic sac, which generally leads to fewer growth complications.
Unequal sharing of placental resources in monochorionic pregnancies can lead to selective fetal growth restriction (sFGR), where one twin grows significantly smaller than the other. This condition is diagnosed if one twin’s estimated weight falls below the 10th percentile and there is a more than 20 percent weight difference between the twins. Maternal health, including nutrition and overall well-being, impacts fetal growth. Slight size differences between twins are common and typically do not indicate a problem, though healthcare providers often recommend more frequent ultrasound monitoring for twin pregnancies to track growth.