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This is a draft cheat sheet. It is a work in progress and is not finished yet.


Dizygotic or fraternal twin - twin fetuses result from fertil­ization of two separate ova
Monozy­gotic or identical twin - twins arise from a single fertilized ovum
Genesis of Monozy­gotic Twins
First 72 hours→ diamni­onic, dichor­ionic
4th-8th day → diamni­onic, monoch­orionic
8th-12th day → monoam­nionic, monoch­orionic
>13 day→ conjoined twin
Superf­etation - requires ovulation and fertil­ization during the course of an establ­ished pregnancy
Superf­ecu­ndation - refers to fertil­ization of two ova within the same menstrual cycle but not at the same coitus, nor necess­arily by sperm from the same male

Mechanism of monozy­gotic twinning


ART (2-5 fold increase) - Monozy­gotic twins
Different races and ethnic groups
Maternal age
Increasing parity
Family history - maternal < paternal
Greater nutrit­ional status reflected by maternal size
Pituitary gonado­tropin in amount greater than usual
Ovulation induction with FSH plus hCG or clomiphene citrate
In vitro fertil­ization (IVF)


Higher serum B-hCG levels → nausea and vomiting
Blood volume expansion is greater (50-60%)
Greater blood loss with vaginal delivery
Greater iron and folate requir­ements
Augmented cardiac output
Lower vascular resistance
Uterine growth is greater


Clinical evaluation
- Uterine size is larger during the second trimester
- Palpating two fetal heads in different uterine quadrants
- Palpating two fetal heads in different uterine quadrants
- Used to determine fetal number, estimated gestat­ional age, chorio­nicity, and amnion­icity
- Separate gestat­ional sacs
Other diagnostic aids
- Abdominal radiog­raphy
- Serum and urine B-hCG and maternal serum AFP


Sonograms of first-­tri­mester twins. A. Dichor­ionic diamnionic twin pregnancy at 6 weeks’ gestation. Note the thick dividing chorion (yellow arrow). One of the yolk sacs is indicated (blue arrow). B. Monoch­orionic diamnionic twin pregnancy at 8 weeks’ gestation. Note the thin amnion encircling each embryo, resulting in a thin dividing membrane (blue arrow).



Prevent preterm delivery of markedly immature neonates
Prenatal visit every 2 weeks beginning at 22 weeks
- 37-57 lbs weight gain – normal BMI
- 40-45 kcal/kg/d - daily recomm­ended augmented caloric intake
- Serial sonogr­aphic examin­ations
- Oligoh­ydr­amnios → uterop­lac­ental pathology
- Single deepest vertical pocket is measured
- <2cm = oligoh­ydr­amnios
- >8 cm hydramnios
Antepartum fetal survei­llance
- Biophy­sical profile
- Nonstress test
- Umbilical artery doppler veloci­metry


Prediction of preterm birth
- Cervical length
- <20mm most accurate for predicting birth before 34 weeks
- Closed internal os by examin­ation was found to as predictive of postponed delivery
- Fetal bronectin levels also may predict preterm birth
- Limited physical activity, early work leave, more frequent health­-care visits, serial sonogr­aphic examin­ations, and structured maternal education regarding preterm delivery risks have been advocated to reduce preterm birth rates in women with multiple fetuses
- Prophy­lactic Tocolysis
treatment did not reduce the rate of twins delivering before 37 or before 34 weeks’ gestation
oral terbut­aline*
- Proges­terone Therapy
17-alp­ha-­hyd­rox­ypr­oge­sterone caproate (17-OHPC) injections are not effective for multifetal gestations
*Vaginal micronized proges­terone - ineffe­ctive for preterm birth prevention before 34 weeks
- Cervical Cerclage
Prophy­lactic cerclage - does not improve perinatal outcome in women with twin pregna­ncies
Physical examin­ati­on–­ind­icated cerclage in women with a second­-tr­imester twin gestation and a dilated cervix may be beneficial
- Pessary
not recomm­ended
- Tocolytic therapy to help halt preterm labor in multifetal pregnancy
does not measurably improve neonatal outcomes
entails higher maternal risk than in singleton pregnancy
augmented pregna­ncy­-in­duced hyperv­olemia, which raises cardiac demands and increases the suscep­tib­ility to iatrogenic pulmonary edema
- Antenatal cortic­ost­eroids
for fetal lung maturation
betame­thasone - reduced rates of perive­ntr­icular hemorrhage
Preterm Prelabor Membrane Rupture
- PPROM after 24 weeks’ gestation, the median number of days to subsequent delivery was 4 days for twins compared with 7 days for singletons
Delayed Delivery of Second Twin
- If delayed delivery is attempted, counseling should include the potential for serious maternal compli­cat­ions.
- The range of gestat­ional age for which benefits outweigh the risks for delayed delivery is likely narrow, and gestations of 22 to 24 weeks would seem the most probable to benefit


At any time during multifetal pregnancy, one or more fetuses may die, either simult­ane­ously or sequen­tially
Causes and incidence of fetal death are related to
1) fetal anomaly
2) chorio­nicity - monoch­orionic twins suffer higher rates of sFGR, TTTS or TAPS from unequal vascular anasto­moses; monoam­nionic pairs can die from cord entang­lement
Early death - vanishing twin
Fetus compressus - in a more slightly advanced gestation, the dead fetus may become compressed apprec­iably
fetus papyraceus - flattened dead fetus through desicc­ation
- decisions should be based on gestat­ional age, the cause of death, and the risk to the surviving fetu
- If the loss occurs after the first trimester, the risk of death or damage to the survivor is largely limited to monoch­orionic twin gestat­ions. Morbidity in the monoch­orionic twin survivor is almost always due to vascular anasto­moses, which often cause the demise of one twin followed by sudden hypote­nsion in the other.
- Single fetal death during the late second and early third trimesters presents the greatest risk to the surviving twin.
If death of one dichor­ionic twin is due to a discordant congenital anomaly in the first trimester, it should not affect the surviving twin
If one fetus of a monoch­orionic twin gestation dies after the first trimester but before viability, pregnancy termin­ation can be considered
Delivery generally occurs within 3 weeks of diagnosis of fetal demise, thus antenatal cortic­ost­eroids for survivor lung maturity should be consid­ered. Regard­less, unless the intrau­terine enviro­nment is hostile, the goal is to prolong the preterm pregnancy.
Dichor­ionic twins can probably be safely delivered at term
Monoch­orionic twin gestations are more difficult to manage and are often delivered between 34 and 37 weeks' gestation


Fetal presen­tations are best described sonogr­aph­ically
Possible presen­tation combin­ations:
-most common: cephal­ic-­cep­halic, cephal­ic-­breech, and cephal­ic-­tra­nsverse


Vaginal vs CS
Cephal­ic-­cep­halic - vaginal birth
Cephal­ic-­non­cep­halic - caesarean section or vaginal
Breech first twin - caesarean section


No evidence currently suggests an increased risk of uterine rupture, and women with twins and one previous cesarean delivery with a low transverse incision may be considered candidates for trial of labor


Following delivery of the first twin, the presenting part of the second twin, its size, and its relati­onship to the birth canal should be quickly and careully ascert­ained by combined abdominal, vaginal, and at times, intrau­terine examin­ation
- Sonography is a valuable aid
- If the fetal head or the breech is fixed in the birth canal, moderate fundal pressure is applied and membranes are ruptured
- Immedi­ately afterward, digital examin­ation of the cervix is repeated to exclude cord prolapse
- Labor is allowed to resume
- If contra­ctions do not begin within approx­imately 10 minutes, dilute oxytocin may be used to stimulate contra­ctions
The safest interval between delivery of the first and second twins was frequently cited as <30 minutes
If the occiput or breech presents immedi­ately over the pelvic inlet, but is not fixed in the birth canal, the presenting part can often be guided into the pelvis by one hand in the vagina, while a second hand on the uterine fundus exerts moderate pressure caudally. A presenting shoulder may be gently converted into a cephalic presen­tation. Altern­ati­vely, with abdominal manipu­lation, an assistant can guide the presenting part into the pelvis
- Sonography can aid guidance and allow heart rate monito­ring. Intrap­artum external version of a noncep­halic second twin has also been described.
If the occiput or breech is not over the pelvic inlet and cannot be so positioned by gentle pressure or if apprec­iable uterine bleeding develops, delivery of the second twin can be proble­matic.
To take maximum advantage of the dilated cervix before the uterus contracts and the cervix retracts, delay should be avoided
Prompt cesarean delivery of the second fetus is preferred if no one present is skilled in the perfor­mance of internal podalic version or if anesthesia that will provide effective uterine relaxation is not immedi­ately available.


Active labor with twins progresses more slowly in both nulliparas and multiparas compared with that in singleton
Second­-stage labor of the first twin also is longer
labor augmen­tation - oxytocin may be used provided that women with twins meet all criteria for its admini­str­ation
labor induction -oxytocin alone or in combin­ation with cervical ripening can safely be used in twin gestations
Augmen­tation or induction of labor is not recomm­ended in multifetal pregnancy


Epidural anesthesia - ideal because it provides excellent pain relief and can be rapidly extended cephalad if internal podalic version or cesarean delivery is required.
General anesthesia - necessary for intrau­terine manipu­lation during vaginal birth (under­liverd twin), uterine relaxation can be accomp­lished rapidly with haloge­nated inhalation agent
nitrog­lycerin (IV/su­bli­ngual) or terbut­aline (IV) - for uterine relaxation


Fetal heart rate monitoring during labor with triplet pregna­ncies is challe­nging
-With vaginal delivery, the first neonate is usually born with little or no manipu­lation
-Subse­quent fetuses, however, are delivered according to the presenting part
--often requires compli­cated obstet­rical maneuvers such as total breech extraction with or without internal podalic version or even cesarean delivery
-Assoc­iated with malpos­ition of fetuses is an increased incidence of cord prolapse
--reduced placental perfusion and hemorrhage from separating placentas are more likely during delivery
Pregna­ncies compli­cated by three or more fetuses should undergo cesarean delivery
-vaginal delivery was associated with a higher perinatal mortality rate
-recom­mended prelabor cesarean delivery for triplets


Sponta­neous Abortion and Vanishing Fetus - monoch­orionic twins have signif­icantly higher early fetal loss rates than dichor­ionic pairs
Congenital Malfor­mations - monoch­orionic twins was almost twice that of dichor­ionic twin gestat­ions. The occurrence risk and concor­dance were substa­ntially higher among monoch­orionic twins
Low Birthw­eight - Multifetal gestations are more likely to be low birthw­eight than singleton pregna­ncies due to restricted fetal growth and preterm delivery. Birthw­eights in twins closely paralleled those of singletons until 28 to 30 weeks’ gestation. Beginning at 35 to 36 weeks’ gestation, twin birthw­eights clearly diverge from those of single­tons. Thus, abnormal growth should be diagnosed only when fetal size is less than expected for multifetal gestation. To confirm suitable growth in dichor­ionic pairs, perform sonography every 4 weeks, starting at 16 to 20 weeks. Monoch­orionic twins are imaged every 2 weeks for twin-twin transf­usion syndrome.
Hypert­ension - those with multifetal gestations are more likely to develop a pregna­ncy­-as­soc­iated hypert­ensive disorder, and the incidence further rises with advancing fetal number


Monoam­nionic Twins
Fetal loss - fetuses alive before 16 weeks’ gestation, less than half survive to the neonatal period. Fetal abnorm­alities and sponta­neous miscar­riage contribute to most loss. High perinatal death is attrib­utable to preterm birth, twin-twin transf­usion syndrome, cord entang­lement, birth weight discor­dance, and congenital anomalies.
Congenital anomaly - concor­dance anomalies - fetal echoca­rdi­ography is indicated
Twin-twin transf­usion syndrome - lower in monoam­nionic twins than in monoch­orionic diamnionic pregna­ncies
Umbilical cord entang­lement - a frequent event. Mothers with monoam­nionic twins are often admitted at 24-28 weeks gestation to begin 1 hour of daily fetal heart rate monito­ring. Betame­thasone is given to promote pulmonary lung matura­tion. If fetal testing remains reassuring and no other interv­ening indica­tions arise, CS is performed at 32-34 weeks gestation.
Unique and Aberrant Twinning
Conjoined Twins - referred to as Siamese twins, joining of the twins may begin at either pole and produce charac­ter­istic forms depending on which body parts are joined or shared, thorac­opagus is the most common. Frequently identified using sonography (fetal poles are seen to be closely associated and do not change relative position from one another, more than 1 three vessels in the umbilical cord, fewer limbs than expected, spine hyperf­lexion, bifid fetal pole, and increased nuchal thickness) at midpre­gSu­rgical separation of an almost completely joined twin pair may be successful if essential organs are not shared. Viable conjoined twins should be delivered by cesare­ann­ancy.
External Parasitic Twins and Fetus-­in-fetu - attached to a relatively normal twin, an external parasitic twin is grossly defective fetus or merely fetal parts. Usually consists of supern­umerary limbs, often with some viscera. A functional heart or brain is absent. Parasitic twins are believed to derived from a dead defective twin, whose surviving tissue attached to and receives vascular support from the normal co-twin.
Fetus-­in-fetu - one embryo may enfold early within its co- twin and mainly intraa­bdo­min­ally. Vertebral or axial bones are found in the fetiform mass, whereas a heart and brain are absent.
Monoch­orionic Twins and Vascular Anasto­moses - All monoch­orionic placentas likely share some anasto­motic connec­tions (excep­tions: anasto­moses between twins are unique to monoch­orionic twin placentas, number, size, and direction of these seemingly haphazard connec­tions vary markedly.
Artery­-to­-artery anasto­moses are most frequent.
Deep artery­-to­-vein commun­ica­tions can extend through the capillary bed of a given villus -> create a common villous compar­tment or "­third circul­ati­on" that has been identified in approx­imately half of monoch­orionic twin placentas
This chronic fetofetal transf­usion may result in several clinical syndromes that include twin-twin transf­usion syndrome (TTTS), twin anemia poycyt­hemia sequence (TAPS), and acardiac twinning
Twin-Twin Transf­usion Syndrome
- blood is transfused from a donor twin to its recipient sibling such that the donor may eventually become anemic and its growth may be restri­cted.
The recipient becomes polycy­themic may develop circul­atory overload with heart failure manisfest as hydrops.
- donor twin is smaller and pale, recipient twin is larger and has volume excess.
- recipient may suffer from hyperv­isc­osity and occlusive compli­cat­ions, polycy­themia may lead to hyperv­isc­osity and occlusive compli­cat­ions.
- Chronic TTS: unidir­ect­ional arteri­ovenous anasto­moses.
-TTTS typically presents in midpre­gnancy when the donor fetus becomes oliguric from hypovo­lemia and decreased renal perfusion.
- Donor develops oligoh­ydr­amnios; recipient develops severe hydramnios (due to inc. urine produc­tion)
- absent amnionic fluid in the donor sac = prevents fetal movement refereed to as "­stuck twin or polyhy­dra­mni­os-­oli­goh­ydr­amnios syndrome - poly-o­li"
Fetal Brain Damage
Cerebral palsy, microc­ephaly, porenc­ephaly, and multic­ystic enceph­alo­malacia are serious compli­cations associated with placental vascular anasto­moses in multifetal gestation.
--> likely caused by ischemic necrosis leading to cavitary brain lesions
donor twin (hypot­ension, anemia, or both --> ischemia) o recipient twin (blood pressure instab­ility and episodes of profound hypote­nsion -->­isc­hemia)
TTTS is diagnosed based on two sonogr­aphic criteria:
1) a monoch­orionic diamnionic pregnancy is identified
2) hydramnios (defined by a largest vertical pocket >8cm) in one sac and oligoh­ydr­amnios (defined by a largest vertical pocket <2cm) in the other twin is found
Sonogr­aphic survei­llance of pregna­ncies at risk for TTTS is recomm­ended:
-begin at approx­imately 16 weeks AOG
- subsequent studies are considered every 2 weeks
Quintero (1999) Staging System
Stage I - discordant amnionic fluid volumes as described in the earlier paragraph, but urine is still visible sonogr­aph­ically within the bladder of the donor twin
Stage II - criteria of stage I, but urine is not visible within the donor bladder
Stage III - criteria of stage II and abnormal Doppler studies of the umbilical artery, ductus venosus, or umbilical vein
Stage IV - ascites or frank hydrops in either twin
Stage V - demise of either fetus
-myoca­rdial perfor­mance index (MPI) or Tei index --> a Doppler index of ventri­cular function calculated for each ventricle
Management and Prognosis
Prognosis is related to Quintero stage and gestat­ional age at presen­tation.
Several therapies are available for TTTS and include amnior­edu­ction, laser ablation of vascular placental anasto­moses, selective feticide, and septostomy
- amnior­edu­ction - needle drainage of excess amnionic fluid
- Septostomy - intent­ionally creating a hole in the dividing amnionic membrane
laser ablation of anasto­moses is preferred for severe TTTS (stages II-IV)
- Selective fetal reduction has generally been considered if severe amnionic fluid and growth distur­bances develop before 20 weeks --> both fetuses typically will die without interv­ention
Twin Anemia­-Po­lyc­ythemia Sequence
This form of chronic fetofetal transf­usion is charac­terized by signif­icant hemoglobin differ­ences between donor and recipient twins.
It is diagnosed antena­tally by MCA peak systolic velocity (PSV) >1.5 multiples of the median (MoM) in the donor and <1.0MoM in the recipient twin.
Sponta­neous TAPS usually occurs after 26 weeks' gestation, and iatrogenic TAPS develops within 5 weeks of a procedure.
Twin Revers­ed-­Art­eri­al-­Per­fusion Sequence aka acardiac twin
a rare but serious compli­cation of monoch­orionic multifetal gestation where there is a normally formed donor twin that shows features of heart failure and a recipient twin that lacks a heart (acardius) and other struct­ures.
- caused by a large artery­-to­-artery placental shunt, often also accomp­anied by a vein-t­o-vein shunt
- failed head growth is called acardius acephalus
- failed head growth is called acardius acephalus
- failure of any recogn­izable structure to form is acardius amorphous
- Radiof­req­uency ablation (RFA) of the umbilical cord is the preferred modality of therapy
Median gestat­ional age at delivery was 37 weeks and the average gestat­ional age at the time of the RFA was 20 weeks
- Major compli­cations were premat­urely ruptured membranes and preterm birth.
Hydati­diform Mole with Coexisting Normal Fetus
This unique gestation contains one normal fetus, and its cotwin is a complete molar pregnancy
- must be differ­ent­iated from a partial molar pregnancy, in which an anomalous singleton fetus (usually triploid) is accomp­anied by molar tissue
Diagnosis is usually made in the first half of pregnancy
- Sonogr­aph­ically, a normal­-ap­pearing twin is accomp­anied by its cotwin (a large placenta containing multiple small anechoic cysts).
Often, these pregna­ncies are termin­ated, but pregnancy contin­uation is increa­singly adopted.
1) the pregnancy prognosis is not as poor as previously thought
2) the risk of persistent tropho­blastic disease is similar whether the pregnancy is terminated or not
Compli­cations of expectant management include vaginal bleeding, hypere­mesis gravid­arum, thyrot­oxi­cosis, and early-­onset preecl­ampsia.