What does double sheep double wool twins mean?

With the use of ovulation-stimulating drugs and the advancement of in vitro fertilization, the incidence of multiple pregnancies has continued to rise. Studies have found that the perinatal mortality rate in multiple pregnancies is significantly higher than that in single pregnancies (about 5 times), and the key reason is premature birth and its related complications. Among them, chorionicity and amnioticity have a great correlation with pregnancy prognosis. At present, only ultrasound can make a diagnosis before delivery. So what do double sheep double wool twins mean?

Twin pregnancies account for about 1% of all pregnancies. There are two types of twins: dizygotic twins and monozygotic twins. The incidence of dizygotic twins is about 2/3 of the number of twins, and the incidence of monozygotic twins is about 1/3 of the number of twins. Dizygotic twins refer to the fact that during one menstrual cycle, two uteruses develop at the same time, and the oocytes of the two eggs are fertilized and implanted separately to grow into two fetuses, also known as dichorionic diamniotic twins (DCDA). Monozygotic twins refer to two fetuses that are produced after one egg cell is fertilized and disintegrates. The genetic genes of the two fetuses are completely identical. Monozygotic twins can be divided into: dichorionic diamniotic twins (DCDA) produced by the disintegration of the blastomere, monochorionic diamniotic twins (MCDA) produced by the disintegration of the inner cell mass, and monochorionic monoamniotic twins (MCMA) produced by the disintegration of the embryonic disc into two primitive streaks.

Types of twins:

1. Dichorionic diamniotic twins (DCDA)

The earlier the gestational sac is, the more clearly it can show the two separated gestational sacs. As the pregnancy progresses, after 7 to 8 weeks, the gestational sac becomes larger and larger and becomes aligned with each other. The strongly echoing chorionic ring becomes thicker and thicker at the basal decidua, and the embryo is gradually formed; the other part becomes softer and finally forms a smooth chorion. If the implantation positions of the two blastomeres are far apart, two separate embryos can be seen on the sonogram later, attached to different positions of the uterine wall; if the implantation positions of the two blastomeres are close, the two embryos can merge into one, but a "triangle" structure is formed at the place where the embryos merge, with the top protruding toward the amniotic cavity and connected to the fetal membrane between them. This protrusion is called the "lambda suture cusp" or "twin peak", as shown in Figure 2. The "twin peak" represents the fusion of two embryos. After 14 weeks, the "twin peak" will become less and less obvious, so the best pregnancy week to determine chorionicity is before 14 weeks.

In addition, if the twins are of different sexes, they are more clearly called dichorionic diamniotic twins, and they can also be definitely dizygotic twins.

2. Monochorionic diamniotic twins (MCDA)

In the early stage, only one gestational sac was seen on the sonogram, and then two yolk sacs, two amniotic sacs and two embryos appeared in one gestational sac. Before the chorionic cavity disappears, two amniotic sacs can sometimes be seen suspended in the chorionic cavity, but because the walls of the amniotic sacs are extremely thin, they are not easy to observe. Afterwards, the two amniotic sacs are close to each other, with no "twin peak" at the intersection with the embryo, forming a "T" shape, as shown in Figure 3.

The diaphragm between MCDA twins is thinner because only two layers of amniotic membrane can be connected to each other. Moreover, the gender of the two fetuses must be the same.

3. Monochorionic monoamniotic twins (MCMA)

In the early stages of pregnancy, only one gestational sac and one embryo are seen. When the embryo buds appear at 6 weeks of pregnancy, the two embryo buds are close to each other, which can easily be misdiagnosed as a single fetus. As the amniotic sac becomes larger, the two fetuses are relatively separated. There is no amniotic membrane between MCMA twins, so the umbilical cords of the two fetuses can be entangled with each other. If the coil is tightly wrapped and blood is blocked, the fetus may die in the uterine cavity. In addition, the gender of the two fetuses must be the same.