MFM CASES |

Multifetal Gestation

OBGYN Clerkship Session Level Objectives

SLO 11: Multifetal Gestation

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• Differentiate the features of Monozygotic, Dizygotic and Multi-Zygotic gestation.
• Counsel a patient on the complications of multifetal gestation.
• Discuss enhanced monitoring requirements of a multifetal gestation compared to a singleton pregnancy.

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BACKGROUND | Definitions

ZYGOSITY

The number of distinct embryos

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CHORIONICITY

The number of placentas

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AMNIONICITY

The number of distinct amniotic sacs

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DISCUSS…

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What do the colloquial terms “identical” versus “fraternal” twins refer to, medically speaking?

WHAT IS THE DIFFERENCE?

MONOZYGOTIC TWINS

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• A single egg fertilized by a single sperm.

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• The union forms a zygote that divides in two, forming two identical embryos.

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• Which results in two siblings always of the same sex and DNA.

DIZYGOTIC TWINS

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• Two different eggs fertilized by different sperm.

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• Each union forms its own zygote and these do not divide, they only become two embryos.

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• Siblings of different or equal sex and same DNA at 50%.

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QUESTION

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Dizygotic twins share which of the following?

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1. The same DNA
2. The same sex
3. The same placenta
4. The same parents

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QUESTION

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Dizygotic twins share which of the following?

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• The same DNA
• The same sex
• The same placenta
• The same parents

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The correct answer is A. Dizygotic twins , or fraternal twins, share, on average, 50% of their DNA, which is the same genetic similarity as non-twin siblings

CAUSES OF TWINNING | THEORIES

MONOZYGOTIC TWINS

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• Slight increase with maternal age.

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• Typically random.

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• May increase with Artificial Reproductive Technologies. e.g. IVF.

DIZYGOTIC TWINS

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• Double ovulation – May be inherited.

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• Increases with maternal age up to 35y.

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• Hormonal induction. e.g. Clomiphene, Letrozole.

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MONOZYGOTIC TWINNING | Definitions

Monozygotic Dichorionic

Identical twins that come from a single fertilized egg which splits early in development. 1-3 days.

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Monochorionic Diamniotic (MCDA)

Genetically identical twins that share a single placenta, but develop in separate amniotic sacs. 4-8 days.

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Monochorionic Monoamniotic (MCMA) “Mo-Mo Twins”

Rare identical twins who share the same amniotic sac and placenta within the uterus. 8-12 days.

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Conjoined Twins

A rare form of Monozygotic (identical) twins who are born physically connected to each other. 13 days or greater.

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MONOZYGOTIC TWINNING

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MONOZYGOTIC EMBRYO SPLITTING

THE EARLIER YOU SPLIT, THE LESS YOU SHARE

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CLINICAL SIGNS OF MULTIPLE PREGNANCY

• Significant hyperemesis
• Faster-than-expected weight gain
• More intense breast tenderness
• Fetal movement felt at earlier gestation and/or movement felt in different parts of the abdomen simultaneously.
• Multiple FHRs auscultated.
• Large for dates.
• Multiple fetal parts on clinical exam

LAB

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• Higher B-HCG for gestational age
• Higher Placental Analytes(e.g. PAPPA, MSAFP, etc.) for gestational age.

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B-HCG Rule of 10S - SINGLETON

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• 10 IU at the time of missed menses
• 100,000 at 10 weeks gestational age (PEAK)
• 10,000 at term

QUESTION

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What are some of the physiological changes that occur in pregnancy, but are more exaggerated with twin pregnancy?

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• Increase of cardiac output

• Hypervolemia

• Insulin Resistance

DIAGNOSIS OF MULTIPLE PREGNANCY

Ultrasound is required to make a diagnosis.

• Ideally between 11-14 weeks
• Number of placental masses
• Thickness membrane dividing sacs (> 2mm)
• Lambda sign = Dichorionic, Diamniotic.
• T sign = Monochorionic, Diamniotic
• Presence of intervening membrane
• If no membrane between two pregnancies = Monochorionic, Monoamniotic.
• Fetal sex

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DICHORIONIC PREGNANCY

• Thick layer of chorion can be identified around each gestational sac.
• Thin amnion seen around each sac
• Lambda sign

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MONOCHORIONIC DIAMNIOTIC PREGNANCY

• Thin amnion encircles each fetus
• Chorion surrounds both fetuses together
• T sign

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THE TRUTH ABOUT MULTIPLES

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Twin, triplet and higher order multiple pregnancies are not “fun” for the patient or Obstetrician.

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Multifetal pregnancy is complicated and carries increased obstetrical and fetal risks.

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MULTIFETAL PREGNANCY RISKS

• Rate of Spontaneous Abortion
• Congenital Anomalies (x2)
• Iron Deficiency Anemia
• Hyperemesis Gravidarum
• Gestational Hypertension
• Gestational Diabetes
• Acute Fatty Liver of Pregnancy
• Pruritic Urticarial Papules and Plaques of Pregnancy (PuPPPs)
• Venous Thromboembolism

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• Preterm Birth (both preterm labour and iatrogenic) – 40-50% of twin pregnancies
• Low Birthweight
• Placental Abruption
• Operative Delivery
• Postpartum Hemorrhage

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Multifetal gestation increases pregnancy complications and risk.

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Risk of Fetal Mortality

• Monoamniotic twins – 50-60%
• Monochorionic diamniotic twins – 25%
• Dichorionic twins – 9%

MONOCHORIONIC PREGNANCY: POTENTIAL COMPLICATIONS

Large anastomoses exist with an artery connected to a vein on the surface of the placenta.

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These are large arteriovenous connections which transfer nutrients and red cells from the donor to the recipient.

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This may result in the twin oligohydramnios-polyhydramnios sequence.

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Occurs in 10-15% of MCDA.

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Different treatment options, such as laser ablation

Twin to Twin Transfusion Syndrome

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MONOCHORIONIC PREGNANCY: POTENTIAL COMPLICATIONS

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Risks Specific to Monochorionic Twin Pregnancies

MONOCHORIONIC TWIN COMPLICATIONS

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⇐ TAPS

sFGR ⇒

TRAP

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QUESTION

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What is this unique complication?

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Cord entanglement can only occur in which type of twins?

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PATIENT MANAGEMENT

How should you manage patients?

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Some centres consider Dichorionic/Diamniotic Twins as a moderate risk pregnancy and can be followed by Midwifery or Family Medicine, unless there are complications.

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At other centres, all multiples are considered to be high risk, and are to be followed by an Obstetrician.

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Monochorionic pregnancies are often very high risk, and require the expertise of a Maternal-Fetal Medicine (MFM) specialist.

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PATIENT MANAGEMENT

General Considerations

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• Diet (increase Iron 30mg/day, Folic Acid 1mg/day, extra 300kcal/day compared to singleton pregnancy).
• Counsel on maternal and fetal complications
• offer Prenatal Aneuploidy screen (unique for multiples) and chronionicity ultrasound (10-14 weeks).
• Anatomy scan (and cervix length) at 18-22 weeks (+ fetal ECHO if Monochorionic)
• Monochorionic ultrasounds every 2 weeks starting at 16 weeks.
• Dichorionic ultrasounds every 3-4 weeks starting at 24-25 weeks
• Work modification at 20-24 weeks can be considered.

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QUESTION

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The diagnosis of twin pregnancy was missed at the patient’s dating ultrasound. She is now presenting for her anatomy scan and 2 fetuses are seen. It is too late to clearly see any insertion of the amnion (Lambda or T-sign) and therefore this cannot be used for diagnosis. The technologist can see clearly, however, that one fetus has male sex genitalia and one has female sex genitalia. This twin pregnancy is:

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1. Monochorionic Monoamniotic
2. Monochorionic Diamniotic
3. Dichorionic Diamniotic
4. Dizygotic diamniotic

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PATIENT MANAGEMENT

Dichorionic Diamniotic Twins

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• Determine chorionicity at 11 - 14 weeks
• Offer prenatal screening
• FTS with Nuchal Translucency at 11-13+6 weeks
• NIPS > 10 weeks
• Anatomy ultrasound at 18 - 22 weeks
• Growth surveillance ultrasound q4 weeks
• Cervical length assessment at time of anatomy scan, continue if risk factors for preterm birth
• Delivery 37 - 38 weeks

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PATIENT MANAGEMENT

Monochorionic Diamniotic Twins

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• Determine chorionicity at 11 - 14 weeks
• Offer prenatal screening
• FTS with Nuchal Translucency at 11-13+6 weeks
• NIPS > 10 weeks
• Surveillance for TTTS/Growth starts at 16 weeks
• Anatomy ultrasound at 18 - 22 weeks
• Fetal Echo 18 - 22 weeks
• Growth surveillance ultrasound q2 weeks
• Cervical length assessment at time of anatomy scan, continue if risk factors for preterm birth
• Delivery 36 - 37 weeks

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PATIENT MANAGEMENT

Monochorionic Monoamniotic Twins

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• Determine chorionicity at 11 - 14 weeks
• Offer prenatal screening
• FTS with Nuchal Translucency at 11-13+6 weeks
• NIPS > 10 weeks
• Surveillance for TTTS/Growth starts at 16 weeks
• Anatomy ultrasound at 18 - 22 weeks
• Fetal Echo 18 - 22 weeks
• Growth surveillance ultrasound q2 weeks
• Increased daily fetal monitoring starting at 24-26 weeks (inpatient can be considered)
• Administer steroid prophylaxis prior to delivery
• Delivery by cesarean section 32 - 33 weeks

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CASE

What do you want to know on history?

• Targeted HPI question
• HINT: Think risk factors, clinical signs/symptoms

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What will you look for on physical examination?

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What will you find different for investigations?

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• 27-Year-Old
• Gravida(G): 1
• Para (P): 0
• Gestational Age: 14 Weeks
• Presents with a diagnosis of twin pregnancy.

CASE

Dating ultrasound at 7-weeks noted two distinct fetal heart rates. Repeat ultrasound booked at 12-weeks to determine number of placentas.

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Conclusion of ultrasound:

2 placentas and 2 amniotic sacs.

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What type of twins is this?

1. Monochorionic Monoamniotic Twins (MCMA)
2. Monochorionic Diamniotic Twins (MCDA)
3. Dichorionic Diamniotic Twins (DCDA)

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This means they are?

1. Monozygotic
2. Dizygotic
3. Could be either

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• 27-Year-Old
• Gravida(G): 1
• Para (P): 0
• Gestational Age: 14 Weeks
• Presents with a diagnosis of twin pregnancy.

CASE

If Monozygotic, splitting of the embryo occurred at ______ days

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If Dizygotic, there were always two distinct embryos.

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The patient states she has had ‘wicked’ morning sickness and her BhCG is 163,000. Other than this she is doing well and your history is unremarkable aside from the ultrasound diagnosis of twin pregnancy.

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The patient states: “WOW! Does this mean they are identical?”

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What is your response?

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• 27-Year-Old
• Gravida(G): 1
• Para (P): 0
• Gestational Age: 14 Weeks
• Presents with a diagnosis of twin pregnancy.

RESOURCES

Interested in learning more? Check out these articles!

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• SOGC#428: Management of dichorionic twin pregnancies (2022)
• SOGC#440: Management of monochorionic twin pregnancies (2023)
• Williams Obstetrics 26e, Chapter 48

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REFERENCES

Authors/Slide Contributors:

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• Dr Sue Chandra
• Dr Sheryl Choo
• Dr Bruno Svajger

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