Placental, Fetal, and Newborn Physiology

Questions

DIRECTIONS (Questions 1 through 29): For each of the multiple choice questions in this section, select the lettered answer that is the one best response in each case.

1. Current research is placing the etiology of certain complications of pregnancy on the formation of a good placenta. To this end, the placental cotyledons are formed primarily by which of the following?

(A) arterial pressure on the chorionic plate and decidua (B) fetal angiogenesis

(E) mesenchymal differentiation secondary to unknown factors

2. Because the fetus is growing rapidly, its need for nutrients and energy exceeds the mother’s on a gram- for-gram basis. Often, the placental transport will achieve a fetal concentration greater than maternal, but occasionally the converse occurs. Which of the following has a lower concentration in the fetus than in the mother?

(A) amino acids (B) iron

3. The placenta is supplied by two umbilical arteries that carry deoxygenated fetal blood. This blood flows into intervillous capillaries and back to the fetus in the single umbilical vein. The maternal

circulation is designed to bathe the placental villi to optimize transport across the placenta of nutrients, oxygen, and metabolic wastes. Which of the following best describes the path of the maternal blood flow?

(A) arteries to placental capillaries to veins (B) arteries to intravillous spaces to veins (C) intravillous spaces to arteries to veins (D) veins to intravillous spaces to arteries (E) veins to placental capillaries to arteries

4. The human placenta is a complex structure that serves as the interface between the fetus and maternal circulation to allow excretory, respiratory, and nutritional functions for the fetus. It does which of the following?

(A) allows mainly small molecules and a few blood cells to pass

(B) allows maternal blood to enter the fetal circulation but not vice versa (C) allows only large molecules to pass

(D) allows total mixing of the maternal and fetal blood

(E) maintains absolute separation between the maternal and fetal circulations

5. A chronic hypertensive patient presents with complaints of decreased fetal movement. Her prenatal care has been sporadic but it appears that she is at 37 weeks’ gestation with an estimated fetal weight of 2,200 g. Concerns are raised regarding placental reserves for oxygenating the fetus. This can be most directly assessed by which of the following?

(A) biophysical profile

(B) a fetal ultrasound growth curve (C) lecithin/sphingomyelin (L/S) ratio (D) maternal alpha-fetoprotein

(E) maternal estriol production

6. Labor is induced at 38 weeks due to severe oligo-hydramnios. The infant is born with a congenital absence of the left hand. This is likely due to which of the following?

(A) amniotic bands (B) chorioangioma

(E) true knots in the umbilical cord

7. A poorly controlled class D diabetic patient desired a repeat cesarean section. An amniocentesis to verify pulmonary maturity was done prior to scheduled surgery at 37 weeks’ gestation. The L/S ratio was 2:1 and phosphatidyl-glycerol was absent. An infant was delivered who developed infant

respiratory distress syndrome (IRDS). What was the most likely reason?

(A) Diabetic patients do not produce lecithin.

(B) Fetal lung maturation may be delayed in maternal diabetes.

(D) The L/S test was done on fetal urine.

(E) Maternal blood was present in the specimen.

8. A patient presents to labor and delivery complaining of regular uterine contractions. Upon reviewing her gestational dating criteria, the following is determined:

Last menstrual period (LMP) places her at 36 weeks’ estimated gestational age (EGA).

Her clinical sizing at her initial obstetrical visit places her at 41 weeks.

Ultrasound done at 10 weeks places her at 38 weeks.

Ultrasound done at this presentation places her at 35 weeks.

Clinical size at presentation places her at 34 weeks.

You determine that she is how many weeks’ EGA?

(A) 34 weeks’ EGA (B) 35 weeks’ EGA (C) 36 weeks’ EGA (D) 38 weeks’ EGA

(E) 41 weeks’ EGA

9. The fetal head is usually the largest part of the infant. Depending on the positioning of the head as it enters the pelvis, labor will progress normally or experience a dystocia due to cephalopelvic

disproportion. The smallest circumference of the normal fetal head corresponds to the plane of which diameter?

(A) biparietal diameter (B) bitemporal diameter (C) occipitofrontal diameter (D) occipitomental diameter

(E) suboccipitobregmatic diameter

10. A patient at her 34-week prenatal visit inquires as to the estimated fetal weight. When told it is likely 4 pounds she gets worried that with only a few more weeks that her fetus is too small and there is a problem. While reassuring her, she is told that most of the growth of an infant is in the last month or two of the pregnancy. During the last month of normal pregnancy, the fetus grows at a rate of

approximately which of the following?

(A) 100 g/wk (B) 250 g/wk (C) 500 g/wk (D) 759 g/ wk (E) 1,000 g/wk

11. A patient is found to be blood type A negative during her first pregnancy. She receives antenatal RhoGAM at 28 weeks. At 32 weeks she develops severe preeclampsia and is induced, resulting in an uncomplicated vaginal delivery. The infant does well and is found to be A positive. The mother is found to have anti-D immunoglobulin at a titer of 1:1. Which of the following best describes how much RhoGAM the patient should receive?

(A) no RhoGAM since she is sensitized (B) mini RhoGAM dose

12. A patient has an emergent cesarean section for an abruption. Because of a large anterior placenta, the placenta was entered during the surgery. The mother is Rh-negative. The infant appears anemic and is Rh-positive. To determine the amount of RhoGAM that needs to be given to prevent sensitization, an estimate of the amount of fetal red blood cells (RBCs) in the maternal circulation is necessary. Fetal RBCs can be distinguished from maternal RBCs by which of the following?

(A) lack of Rh factor

(B) lower amounts of hemoglobin (C) nucleated RBCs

(D) resistance to acid elution (E) shape

13. The oxygen dissociation curve of fetal blood lies to the left of the curve of maternal blood. Which of the following is implied?

(A) At any given O2 tension, fetal hemoglobin (Hb F) binds less O2 than adult hemoglobin (Hb A).

(B) At any given pH, Hb F binds less O2 than Hb A.

(D) O2 should transfer easily to the fetus.

(E) There is more Hb A than Hb F at delivery.

Questions 14 through 17 apply to the following topic:

In utero, the fetus exists in a “water”-filled environment. Oxygen is derived from the placenta. The fetal lungs are filled with amniotic fluid. Given the lower oxygen tension of the fetal blood, the circulation through the heart and lungs is altered to allow optimal oxygen delivery to the most critical structures. Yet this unique circulation must convert in minutes upon delivery to a typical adult circulatory flow.

14. Oxygenated blood from the umbilical vein enters the fetal circulation via which of the following?

(A) ductus arteriosus (B) the inferior vena cava (C) intrahepatic artery (D) the lesser hepatic veins

(E) portal sinus and ductus venosus

15. In the fetus, the most well-oxygenated blood is allowed into the systemic circulation by which of the following?

(A) ductus arteriosus (B) foramen ovale (C) ligamentum teres (D) ligamentum venosum (E) right ventricle

16. In the fetal circulation, the highest oxygen content occurs in which of the following?

(A) aorta

(B) ductus arteriosus (C) ductus venosus (D) superior vena cava (E) umbilical arteries

17. In systemic circulation of the fetus, the highest oxygen content occurs in which of the following?

(A) ascending aorta (B) descending aorta (C) ductus arteriosus (D) left ventricle (E) umbilical vein

18. A couple bring their newborn to the pediatrician with concerns regarding possible bruises on the infant’s leg. There is no evidence of trauma or violence. The couple had an uncomplicated home birth.

Concerns are raised regarding the infant’s coagulation status. Which of the following best describes typical fetal coagulation at birth?

(A) demonstrates a hypercoagulable state

(B) depends solely on platelet activity until well into the first week of life when clotting factors are activated

(E) shows significantly less clotting capabilities than in the adult

19. The fetus can produce immune antibodies during development. Also there is some transport of immune antibodies across the placenta. In the fetal blood at birth (compared to maternal blood), there is/are generally which of the following?

(A) more immunoglobulin G (IgG) (B) similar IgG levels

Questions 20 through 22 apply to the following patient:

A 21-year-old patient who is a G3P0111 at EGA of 33 weeks presents with preterm labor. She had a prior preterm infant at 28 weeks who still has pulmonary dysplasia, so she is very concerned about the pulmonary development of this fetus.

20. Lung surfactant is critical to pulmonary functioning by keeping surface tension in the alveoli low and thereby decreasing the occurrence of atelectasis and atrioventricular (AV) shunting. Surfactant is formed in which of the following?

(A) epithelium of the respiratory bronchi (B) hilum of the lung

(D) type I pneumocytes of the lung alveoli (E) type II pneumocytes of the lung alveoli

21. The presence of which of the following substances is most reassuring that fetal lungs will be mature?

(A) phosphatidylinositol (B) phosphatidylethanolamine (C) phosphatidylglycerol (PG) (D) phosphatidylcholine

(E) phosphatidylinositol deacylase

22. Fetal breathing movements can be an indicator of fetal well-being in utero and should occur in what interval of time?

(A) every 30–60 seconds (B) every 30–60 minutes

(E) every 24 hours associated with fetal movement

23. A patient presents for her routine prenatal visit at 32 weeks’ EGA. Her pregnancy up to now has been uncomplicated. Her BMI is 25. Her laboratory testing is normal including a 1-hour glucose screen. An

anatomic ultrasound done at 22 weeks was normal and confirmed her dating. Her fundal height is 37 cm today. A brief bedside ultrasound reveals an amniotic fluid index (AFI) of 30 cm. Which of the

following situations is most likely to be the etiology of polyhydramnios?

(A) duodenal atresia (B) renal atresia

24. A fetus has an infection that is causing acute hemolysis. At birth, the infant is not jaundiced though the liver is enlarged. The lack of fetal jaundice is because of which of the following?

(A) Fetus has a great capacity for conjugating bilirubin.

(B) Fetus produces biliverdin.

(D) Liver plays no part in fetal blood production.

(E) Unconjugated bilirubin is cleared by the maternal liver.

25. Which of the following ratios best describes the serum insulin and glucose levels in the newborn infant of a poorly controlled diabetic mother in comparison to the newborn infant of a eu-glycemic mother?

26. Which of the following best describes the fetal kidneys?

(A) They are first capable of producing highly concentrated urine at 3 months.

(B) They are first capable of producing highly concentrated urine at 6 months.

(D) If absent, they are associated with pulmonary hypoplasia.

(E) They produce only normotonic urine.

27. A woman presents for her new obstetrical visit at 12 weeks’ EGA. Her medical history is

complicated by Graves thyroiditis that has been treated with radioactive iodine a few years prior. The patient is currently being maintained on thyroid replacement. She is worried that this will compromise the fetus. She is told that the interaction between maternal and fetal physiology relative to thyroid function is complex. Which of the following is an accurate description of this interaction?

(A) Maternal thyroid hormones (T4 and T3) readily cross the placenta.

(B) Maternal thyrotropin easily crosses the placenta.

(D) The fetal thyroid concentrates iodide.

(E) The placenta serves as a barrier to maternal iodine crossing to the fetus.

28. A fetus has genotype 46,XY. Early in embryoge-nesis, the right testis does not form (dysgenesis).

What will be the resulting developments?

(A) development of the right mesonephric duct (B) development of a right ovary

(E) true hermaphrodite

29. A female fetus has partial fusion of the two Müerian ducts and complete failure of septal resorption.

What is the resulting uterine anomaly called?

(A) didelphys uterus (B) Müllerian agenesis (C) septate uterus (D) unicornuate uterus (E) vaginal septum

Answers and Explanations

1. (A) About 6 weeks after conception, the tro-phoblast has 12 to 15 major arteries invading deeply into the myometrium and 20 or more lesser arteries. The pressure generated by these major vessels forces the chorionic plate away from the decidua and forms 12 to 20 cotyledons. Fetal angiogenesis is partly contributory to cotyledon formation as well.

2. (C) Although much of placental transport is passive, a large number of necessary metabolic products are actively transported against a concentration gradient. This accounts for many instances of

nutritional sparing of the fetus even though maternal nutrition is poor. Fetal PO2, however, is significantly less than maternal. This is why Hb F is needed to facilitate transport of the decreased oxygen to the fetal tissue. The average oxygen saturation of intervillous blood is estimated to be 65% to 75% with a partial pressure (PO2) of 30 to 35 mm Hg. The oxygen saturation of umbilical vein blood is similar but with a lower oxygen partial pressure.

3. (B) Oxygenated maternal arterial blood flows into the intervillous spaces, exchanging oxygen with the fetal blood across the placental tissues. It is then collected in maternal veins and reenters the maternal vascular system. The fetal and maternal vascular systems do not normally mix maternal and fetal blood.

The human placenta is a hemochorial placenta.

4. (A) The placenta allows a few maternal and a few fetal cells to cross. There may be as much as 0.1 to 3.0 mL of fetal blood in the maternal circulation normally; however, there is no free passage. The systems should be separate and to the degree to which they are not, maternal sensitization may occur.

5. (A) This is a series of assessments utilizing ultrasound evaluation and a nonstress test. It is the only immediate and direct measure of placental respiratory function and fetal activity listed. Es-triol

changes can be delayed as can a fall-off in fetal growth. L/S ratio is a test for lung maturity of the fetus, not the placental reserve. Maternal serum alpha-fetoprotein (MSAFP) is a prenatal screen for neural tube defects. Although asymmetric intrauterine growth retardation can result from a decreased reserve and be detected by ultrasound, this is not an immediate assessment of status.

6. (A) Amniotic bands can cause severe fetal deformities and even amputations by constricting fetal parts.

They are thought to result when small areas of amnion tear and form tough bands from the resultant scarring with healing. The phenomenon is also associated with oligohydramnios. Placental

abnormalities have also been noted with a full-blown amniotic band syndrome.

7. (B) In diabetic patients, the L/S ratio alone may not be adequate to predict the onset of IRDS, hence the recommendation to assess for the presence of PG as a better indicator of lung maturity. Fetal urine would not have a high L/S ratio. Maternal blood would lower the L/S ratio in most cases, as it has an L/S ratio of about 1.4. Fetal lung biochemical measurements are less reliable predictors of fetal lung maturity in poorly controlled pregnant diabetic patients.

8. (D) Dating a pregnancy is critical to determining the best management. Traditionally, the first day of the LMP is used. However, with the advent of ultrasound, which allows measurement of the fetus, dating has become more accurate. The first ultrasound done during a pregnancy should be compared with the

dating from the LMP. If dating based on the LMP is within the error of the ultrasound, LMP dating is used. The error of the ultrasound is roughly +1 week for the first trimester, +2 weeks for the second trimester, and +3 weeks for the third trimester. Since the initial ultrasound was done in the first

trimester and was 2 weeks different from the LMP, the ultrasound dating is used. Later ultrasounds are not used to change the due date but can be used for estimating the fetal growth rate. Given that the

ultrasound and clinical sizing is smaller than the EGA of 38 weeks determined by the initial ultrasound, concerns of growth restrictions or a constitutionally small fetus can be raised.

9. (E) A vertex presentation offers the smallest circumference of the fetal head to the pelvic passage. The circumference at this point is about 32 cm. At the greatest point of the circumference (the

occipitofrontal diameter), it is about 34 cm. In addition to the circumference, the ability of the fetus to negotiate the pelvic curve is very much dependent on the position of the presenting vertex, with a well- flexed head in the occiput anterior (OA) position being optimal.

10. (B) A good rule of thumb is that the fetus gains one-half pound a week during the last few weeks of gestation. Of course, if placental insufficiency exists, such weight gain does not occur. In uncontrolled diabetes mellitus, classes A, B, and C, growth during this period is accelerated. More severe forms of diabetes (e.g., F, R, or H) may have small-vessel disease with resulting placental insufficiency.

11. (C) RhoGAM consists of anti-D immunoglobin. Since she received RhoGAM only a month earlier, there would still be some detected during the “RhoGAM” workup postpartum. She should still receive 1 ampule of RhoGAM unless testing determines a large fetal-maternal bleed, which is not likely in an uncomplicated vaginal delivery.

12. (D) Small numbers of fetal red cells can be detected in the maternal circulation by the Kleihauer- Betke test, which uses the fetal cells resistant to acid elution for identification. Fetal red cells can exist and function at lower pH values than adult RBCs. Fetal RBCs have less than two-thirds the life span of adult RBCs. Fetal RBCs may also be nucleated. The nucleation helps identify fetal RBCs on a smear.

This nucleation disappears early in normal pregnancy. Fetal cells then closely resemble reticulocytes.

13. (D) The pH of the fetus is slightly lower than the maternal pH. The difference in O2 affinity is very small in vivo. The fetus lives at a lower O2 concentration with an oxygen-loving hemoglobin (Hb F), which allows oxygen transport to fetal tissues despite very low maternal oxygen tension. Although Hb A starts being produced in the third trimester, the Hb F is still the dominant form at delivery.

14. (E) The umbilical vein comes directly from the placenta and distributes highly oxygenated blood to the liver, the portal system, and the inferior vena cava. The umbilical vein enters the fetus and divides immediately into the portal sinus (carrying blood to the hepatic veins) and the ductus venosus (carrying blood to the vena cava). The ductus arteriosus connects the fetal pulmonary vasculature with the fetal aorta (see Figure 7–1).

Figure 7–1. The fetal circulation. (Reproduced, with permission, from Cunningham FG, Levenoe KJ, Bloom S, Hauth JC, Rouse DJ, Spong C. Williams Obstetrics, 23rd ed. New York: McGraw-Hill, 2001.)

15. (B) The foramen ovale lets oxygenated blood into the left side of the heart. The ligaments mentioned are found after birth and represent occluded vessels from the fetal circulation. Before birth; however, they serve as the shunting mechanism that makes fetal oxygenation possible. The ductus arteriosus allows the flow from the right ventricle to enter the systemic circulation after the aortic arch.

16. (C) The fetal venous blood from the placenta has the highest oxygen content. This is true because fetal venous blood in the ductus venosus has most recently received oxygenation. Pulmonary fetal

oxygenation does not occur until the infant’s first breath. The superior vena cava has oxygen-deficit blood. The ductus arteriosus and aorta have mixed blood that eventually goes to umbilical arteries.

17. (E) The umbilical vein always has the highest oxygen concentration. The next highest oxygen concentration in the systemic circulation is in the ascending aorta prior to the insertion of the ductus arteriosus. The arteries supplying the fetal brain branch off that part of the ascending aorta allowing the blood with the highest systemic oxygen levels to be preferentially shunted to the brain. The left

ventricle is not part of the systemic circulation.

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