One weighed more than the other. Odds are that your twins, even though they may be identical, will be different sizes when they arrive. Picture by weatherking. My boys were nearly a pound different when they were born, but one has esophageal atresia. Sally — thanks for sharing your experience with your boys. Save my name, email, and website in this browser for the next time I comment. Estimates and Averages Keep in mind that twins do come really early, typically around 36 weeks, compared to 40 weeks for a singleton pregnancy.
Don't forget to pick up a copy of the definitive guide to raising twins. Click here to learn more about the book and get your copy. We selected cohort-type studies with delayed verification that evaluated the accuracy of biometric measurements at ultrasound scanning of twin pregnancies that had been proposed for the diagnosis of estimated BWD, compared to BWD measurements after birth as a reference standard.
In addition, we only selected studies that considered twin pregnancies and applied a reference standard for EFWD for the target condition of BWD. We screened all titles generated by electronic database searches. Two review authors independently assessed the abstracts of all potentially relevant studies. We included 39 eligible studies with a median study sample size of In terms of risk of bias, there were many unclear statements regarding patient selection, index test and use of proper reference standard.
In terms of applicability, most studies were of low concern. The summary estimate of sensitivity was 0. The diagnostic accuracy did not differ substantially between estimation by abdominal circumference and femur length but femur length had a trend towards higher sensitivity and specificity.
Subgroup analyses were not possible by sex of twins, chorionicity or gestational age due to insufficient data. Study characteristics We searched medical databases to March for studies comparing ultrasound measurements to birth weight differences and we identified 39 studies. Key results We found that ultrasound estimation of fetal weight differences compared to birth weight differences was not reliable. Why is this important? If twins have separate chorion, they will have two separate placentas.
The amnion on the other hand is the innermost fetal membrane. It protects the fetus and includes amniotic fluid. Like the chorion, twins can either share an amnion or have one all of their own.
Since this is the inner layer, the options here depend on the chorion. One chorion that is shared can contain one or two amnions. So even though two babies may share the same placenta, they can each still be swimming in their own amniotic fluid if they have their own amniotic sac.
On the other hand, two chorions mean two separate amnions. Got all that? This means one placenta and one amniotic sac for both babies. Ultrasounds are most accurate at determining chorionicity in the first trimester. With a fraternal twin pregnancy, separate eggs are individually fertilized by two different sperm cells creating two individual zygotes. Each zygote will have its own placenta and amniotic sac. On the other hand, an identical pregnancy begins with the combination of one egg and one sperm cell, but at some point that single egg splits.
When that happens affects the development of the placenta and amniotic sac. For parents who want to know for sure, genetic testing performed after birth will give the clearest answers.
Being pregnant with twins does come with some additional risks. Some complications that may be more likely with a twin birth include:. Of course, knowing which type of twin combination is present during pregnancy is important to understand the level of risk involved. If the babies are sharing a placenta, there is a risk that one baby may receive more of the nutrients and oxygen needed for development. Because there is nothing to separate the babies from one another in the womb there is a risk of cord entanglement.
Whether inside or outside the womb, twin development can be confusing and offer some extra complications. Some questions can be answered via ultrasound, but others can only be discovered through genetic testing.
More types of twins exist than previously thought.
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