A 29-year-old Caucasian primigravida is 20 weeks pregnant with twins. Today, on her routine ultrasound for fetal anatomy, she found out that she is carrying two boys.
In this patient’s case, which of the following statements correctly describes the zygosity of this pregnancy?
The incidence of monozygotic twinning is constant at a rate of one set per 250 births around the world. It is unaffected by race, heredity, age, parity, or infertility agents. The incidence of dizygotic twinning is influenced by all of these factors, and varies based on group. These twins of the same gender could be monozygotic or dizygotic. Two identifiable chorions can occur in monozygotic or dizygotic twinning. Dizygotic twins will always have two amnions and two chorions, since they result from fertilization of two eggs. Therefore, dizygotic twins may be of the same or different genders. The placentas of dizygotic twins may be totally separate, or intimately fused, depending on the location of the implantation of the two zygotes. Monozygotic twins are always of the same gender because they originate from the division of one zygote; however, they may be monochorionic or dichorionic depending on when the separation of the twins occurred. Twenty to thirty percent of monozygotic twins have dichorionic, diamniotic placentation (similar to dizygotic twins), which results from separation of the blastocyst within the first 72 hours after fertilization. Division that occurs between days 4 and 8 will result in monochorionic, diamniotic twins. One percent of monozygotic twins will be monochorionic, monoamniotic, which occurs with division after day 8 but before the embryonic disc is formed. Conjoined twins are always monozygotic, and occur with late division after formation of the embryonic disk.
After delivery of a term newborn with Apgar scores of 2 at 1 minute and 7 at 5 minutes, you ask that blood from the umbilical arteries be collected for pH.
The umbilical arteries carry which of the following?
Deoxygenated fetal blood is returned directly to the placenta through the umbilical branches of the two hypogastric arteries. The umbilical arteries exit through the abdominal wall at the umbilicus and continue by way of the umbilical cord to the placenta. Deoxygenated blood circulates through the placenta then returns, oxygenated, to the fetus via the umbilical vein. The umbilical arteries atrophy and obliterate within 3 to 4 days after birth; remnants are called umbilical ligaments.
A 25-year-old P0 presents for routine anatomy ultrasound at 20 weeks’ gestation. The only significant finding at the time of ultrasound is the presence of a single umbilical artery (SUA).
How should you counsel this patient about the finding of a SUA?
The finding of a SUA occurs in approximately 1% of pregnancies, and 5% of at least one twin. The incidence of SUA is increased in diabetic mothers. The incidence of major fetal malformations when SUA is identified has been reported to be as high as 18%, and usually involves the cardiac or renal systems; therefore, a careful anatomic survey is indicated. The rate of aneuploidy in the setting of isolated SUA is not increased, so routine karyotype analysis is not needed unless there are other indications to offer this testing. In the absence of other findings, SUA is rarely associated with poor pregnancy outcomes.
A 25-year-old P0 presents for routine anatomy ultrasound at 20 weeks’ gestation. The only significant finding at the time of ultrasound is the presence of a single umbilical artery (SUA). Targeted ultrasound does not demonstrate any other abnormalities. The patient asks you if this SUA will impact how you manage the rest of her pregnancy.
What should you tell her?
The finding of SUA in the absence of other abnormalities does not require karyotype evaluation, early delivery, or delivery by cesarean. The timing and mode of delivery may be determined by routine obstetric indications. Patients with a fetus with SUA should undergo periodic growth assessments with ultrasound, as there is an increased risk of growth restriction in these fetuses.
A 22-year-old G1P0 at 28 weeks’ gestation by LMP presents to labor and delivery complaining of decreased fetal movement. She has had no prenatal care. On the fetal monitor there are no contractions. The fetal heart rate is 150 beats per minute and reactive, with no decelerations in the fetal heart tracing. An ultrasound demonstrates a 28-week fetus with normal anatomy and size consistent with menstrual dates. The placenta is implanted on the posterior uterine wall and its margin is well away from the cervix. A succenturiate lobe of the placenta is seen implanted low on the anterior wall of the uterus. Doppler flow studies indicate a blood vessel is traversing the cervix connecting the two lobes.
This patient is most at risk for which of the following?
This patient has a vasa previa. When fetal vessels cross the internal os (vasa previa), rupture of membranes may be accompanied by rupture of a fetal vessel leading to fetal exsanguination. Vasa previa does not increase the risk for placenta accreta or amniotic fluid embolism. With velamentous insertion of the cord, the umbilical vessels separate in the membranes at a distance from the placental margin which they reach surrounded only by amnion. Such insertion occurs in about 1% of singleton gestations but is quite common in multiple pregnancies. Fetal malformations are more common with velamentous insertion of the umbilical cord. An increased risk of premature rupture of membranes and torsion of the umbilical cord has not been described in association with velamentous insertion of the cord.
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