The 28th week of pregnancy marks the entry into the third trimester. For parents expecting twins, this stage often prompts questions about whether their babies are developing on track. Due to the complexities of a multiple gestation, including reduced space and shared resources, monitoring fetal weight becomes a major focus for healthcare providers. Understanding the expected size and the specific factors that influence twin development is important.
Expected Weight Range for Twins at 28 Weeks
The estimated fetal weight (EFW) for a single twin at 28 weeks gestation typically falls within a range of approximately 875 grams to 1.13 kilograms, or about 2.0 to 2.5 pounds. This average is based on specialized twin growth charts, which doctors use instead of standard singleton charts. Twin fetuses normally weigh slightly less than singletons starting in the late second trimester. Around 26 to 28 weeks, the growth curve for twins begins to diverge due to the constraints of sharing the uterine environment. The goal of monitoring is to ensure both babies are following an appropriate growth trajectory, as the rate of growth is a more telling indicator of well-being than the absolute weight.
Key Factors Affecting Twin Growth
The primary factor influencing twin weight is the chorionicity, which describes whether the babies share a placenta. Dichorionic twins, who each have their own separate placenta, typically maintain a growth rate closer to singletons for a longer period. Monochorionic twins, who share one placenta, often experience a divergence in their growth trajectory earlier.
A significant concern is Selective Fetal Growth Restriction (sFGR), where one twin is significantly smaller than the other. This condition is diagnosed when one fetus has an estimated weight below the 10th percentile and there is a 20 to 25% or greater difference in weight between the two babies. In monochorionic pregnancies, sFGR is often caused by an unequal sharing of the common placenta.
Maternal health and nutritional intake also play a substantial role in fetal size. Adequate maternal weight gain, particularly during the second trimester, correlates with a larger estimated fetal weight in the third trimester. Pre-existing maternal conditions such as chronic hypertension or the development of pre-eclampsia can reduce blood flow to the placenta, potentially hindering the growth of one or both fetuses.
Developmental Status of Twins at 28 Weeks
At 28 weeks, the twins have reached a significant milestone where their organs are rapidly maturing in preparation for life outside the womb. This gestational age is often cited as the point when the chance of survival for prematurely born infants improves considerably. The babies are now spending a large amount of time in Rapid Eye Movement (REM) sleep, which reflects the rapid development of their brains.
A critical development at this stage is the maturation of the lungs. The fetuses begin producing pulmonary surfactant, a substance that prevents the air sacs from collapsing and is necessary for independent breathing. Their skeletal structure is firming up as cartilage turns into bone, and they are starting to deposit layers of white fat beneath the skin. This fat deposition helps the babies regulate their body temperature after birth.
While their length growth may start to slow down due to the confined space, the focus shifts to gaining weight. They also begin to establish regular sleep-wake cycles, which parents may notice as distinct periods of movement and rest.
Medical Monitoring and Management Strategies
The monitoring of twin growth at 28 weeks relies heavily on frequent ultrasound examinations. These scans are typically performed every two to four weeks for dichorionic twins and more often for monochorionic twins to track estimated fetal weight and the difference in size between the pair. Umbilical artery Doppler studies are also used to measure blood flow through the umbilical cord, providing information on how well the placenta is functioning. Abnormal Doppler results can indicate placental compromise and necessitate increased surveillance.
In cases of concerning growth restriction or a high risk of preterm delivery, a management strategy may include administering antenatal corticosteroids. These steroid injections are given to the mother between 24 and 34 weeks gestation to accelerate the babies’ lung maturity. This proactive measure significantly improves the respiratory outcomes for twins who may need to be delivered prematurely.