Early Path Medical Consultation Services

Early Path Medical Consultation Services
Pathology Services Working for Safer Pregnancies

Placental Pathology - Medical Student Lecture 2001
Carolyn M. Salafia, MD
Professor of Pathology and Pediatrics

CONTENTS

1. OVERVIEW

2. INTRODUCTION

3. PREECLAMPSIA

4. FETAL GROWTH

5. INTRAUTERINE INFECTION

6. MULTIFETAL PREGNANCY

MULTIFETAL PREGNANCY

Illustrate the anatomy of multiple gestations, and discuss the most common complications

Multiple gestations (Figure 21) carry higher risk to maternal and fetal well-being, including perinatal mortality in 14% of twins; risks vary with number of babies, and with type of placentation. Higher-order multiple births are more frequent in recent years, due to delayed child bearing, use of fertility drugs and in vitro fertilization techniques. Natural occurring higher multiple births are infrequent occurrences (triplets 1/10,000 births, quadruplets 1/100,000 births). Twinning is the most common naturally occurring type of multiple gestation. The incidence of monozygotic twinning (identical or monovular twins) is stable worldwide (3-5/1,000 births). The etiology is unknown but may be a teratogenic event. There is a recurrence risk of 1%. Dizygotic twinning (fraternal or binovular twins) in the United States occurs in approximately 8/1,000 births, but this incidence varies with populations. Familial twinning is due to dizygotic twinning with stronger maternal inheritance. Twinning is more frequent with advanced maternal age and increased parity. All monochorionic twins are monozygotic, 20% of dichorionic twins are also monozygotic. Fusion of dichorionic placentas occurs when the two implantation are close to each other in the uterine lining. Zygosity can be determined in 55% of twins by the combination of placental examination and knowing the sex of the babies. What type of membranes are present (two chorions/two amnions, one chorion/two amnions, one chorion/one amnion) with monozygous twins is determined by when the zygote or blastocyst divides. Dizygous twins arise from two separate zygotes (and therefore do not have equivalent genetic makeup). Two chorions and two amnions are found. About 65% of monozygous twins develop from one zygote by inner cell mass splitting. Because the twins obligatorily share the vascular chorion shell, vascular anastomoses are almost universal. There is always one chorion, and a single shared placental disk. About 35% of monozygous twins develop from one zygote. Blastomeres may separate anywhere from the 2-cell to the morula stage (the latter begins 3 days after fertilization), to produce identical blastocysts. Each blastocyst will develop its own complete (chorion and amnion) shell. In 10% of cases, placentas are completely separate. These monozygous twins can mistakenly labeled dizygous.

Superficial vascular anastomoses, seen in most monochorionic placentas, are easy to identify. Deep anastomoses, the third circulation, are most commonly an artery-to-vein anastomosis balanced with a returning vein-to-artery anastomosis. (Figure 22) Because the blood vessels in the chorion shell develop first independent of the fetal heart, and only later join up with the fetal heart to form a complete circulatory circuit, some vascular sharing is almost universal in monochorionic twins. Cord accidents are involved in 50% of fetal death of monochorionic -monoamniotic twins, and include entanglement in monochorionic -monoamniotic twins or prolapse of the umbilical cord of twin B during delivery of twin A. (Figure 23) During the first trimester, as many as 21% of twins may be lost. In early fetal losses, there may be no evidence of the fetus at the time of delivery, often referred to as the "vanishing twin". A fetus papyraceous (Figure 24) forms following fetal death after the fetus' bones are sufficiently developed to retain the fetal form over months of incubation in a warm and watery environment. Death of one twin may be followed by death of the co-twin, as they share what may be a "hostile environment," or there may be premature delivery of the living twin. While maternal coagulopathy may occur with a retained dead fetus, it is very rare if only one of the pair dies. Evidence of embolic or thrombotic damage in the brain, skin, kidneys, and intestine of the living co-twin may be due to the passage of thromboplastic agents from dead tissue to the surviving twin through placental vascular anastomoses, or to acute hypovolemia/hypotension in the co-twin after sibling demise and drop in blood pressure across anastomoses. The placenta may remain somewhat normal after the fetal death if the living twin can continue to maintain perfusion but the placenta of the demised sibling will eventually atrophy if this is not possible.

Prematurity, acute ascending infection, premature rupture of membranes, and intrauterine growth retardation of one or both twins are all more common in multifetal gestations, and carry risk for immediate and long-term morbidity/mortality. Twin gestations begin with growth equal and appropriate for gestational age, however, uterine crowding, decreased placental reserve, or abnormal blood flow to one twin will result in intrauterine growth retardation of one or both twins. Congenital malformations are more common in monochorionic twins, seen in approximately 10%. Single umbilical artery is seen in approximately 1% of normal deliveries and is seen in 2.5-4% of twins. Single umbilical artery is seen in 20% of acardiac twins. There is a higher incidence of loss of chromosomal material, and an increase in midline defects, including neural tube defects, facial clefts, tracheoesophageal fistulas and sirenomelia. Abnormal cord insertions are seen in 16% of all twins, compared to 1% in singletons. Marginal insertion is seen in 14% of monochorionic and 4% of dichorionic placentas and velamentous insertion is seen in 9% of monochorionic and 5% of dichorionic placentas. (Figure 25) All monochorionic placentas have vascular anastomoses, however only 7.5-30% develop twin transfusion syndrome (TTS). Chronic TTS occurs most often in monochorionic- diamniotic placentas, infrequently in monochorionic-monoamniotic placentas (because of nearly complete sharing of the placental substance), and rarely in dichorionic-diamniotic fused placentas. TTS is caused by unequal blood flow from the artery of one twin to a vascular bed in the placenta that is unidirectionally drained by a vein from the other twin (recipient), the "third circulation". TTS may be diagnosed in monochorionic twins when there is >20% discordance in the fetal weights and > 5 g/dl difference in hemoglobin. The donor is growth retarded, anemic, with delayed organ maturation and oligohydramnios. The recipient becomes plethoric and is overgrown. Polyhydramnios and congestive heart failure due to volume overload and polycythemia may occur. The placenta reflects the fetal abnormalities. Acute TTS may occur at the time of fetal death (when the dead fetus becomes a low resistance system), during delivery, or rarely in conjunction with chronic TTS. (Figure 37 and Figure 38)

Acardia is a very rare malformation restricted to monozygotic twins with an underestimated occurrence of 1/35,000. The origin of acardia is related to reversal of blood flow, most commonly through superficial vascular anastomoses in a monochorionic placenta; 60% of which are diamniotic. The acardiac twin loses direct vascular connection with the placental villi and receives its entire blood supply from pump twin. Close proximity of the two umbilical cords on a common chorion and discordant development between the embryos must be present for reversal of arterial blood flow to occur. Deoxygenated, low-pressure blood from the pump twin, which would normally return to the placenta, instead flows directly to the acardiac twin, resulting in a wide array of structural abnormalities Mortality of the pump twin is 50-75%, usually the result of heart failure, of course mortality of the acardiac is 100%. (Figure 39) Conjoined twins are a rare malformation of monochorionic monoamniotic (monochorionic- monoamniotic) twinning, the result of late division of the blastocyst, with a frequency of 1/50,000-100,000.