Monoamniotic twins are identical or semi-identical

twins Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...

that share the same

amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shor ...

within their mother's

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The uter ...

. Monoamniotic twins are always

monochorionic Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies. ...

and are usually termed Monoamniotic-Monochorionic ("MoMo" or "Mono Mono") twins. They share the

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mater ...

, but have two separate

umbilical cords In placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologic ...

. Monoamniotic twins develop when an embryo does not split until after formation of the amniotic sac, at about 9–13 days after

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

. Monoamniotic triplets or other monoamniotic multiplesMoMo Twins; Monochorionic Monoamniotic Twins
By Pamela Prindle Fierro, About.com. Retrieved on July 9, 2009 are possible, but extremely rare. Other obscure possibilities include multiples sets where monoamniotic twins are part of a larger gestation such as triplets, quadruplets, or more.

Occurrence

Monoamniotic twins are rare, with an occurrence of 1 in 35,000 to 1 in 60,000 pregnancies.

Complications

The survival rate for monoamniotic twins has been shown to be as high as 81% to 95% in 2009 with aggressive fetal monitoring, although previously reported as being between 50% to 60%. Causes of mortality and morbidity include: *

Cord entanglement A nuchal cord is when the umbilical cord becomes wrapped around the fetus's neck. Symptoms present in the baby shortly after birth from a prior nuchal cord may include duskiness of face, facial petechia, and bleeding in the whites of the eye. Com ...

: The close proximity and absence of amniotic membrane separating the two umbilical cords makes it particularly easy for the twins to become entangled in each other's cords, hindering fetal movement and development. Additionally, entanglement may cause one twin to become stuck in the birth canal during

labor Labour or labor may refer to: * Childbirth, the delivery of a baby * Labour (human activity), or work ** Manual labour, physical work ** Wage labour, a socioeconomic relationship between a worker and an employer ** Organized labour and the la ...

and expulsion. Cord entanglement happens to some degree in almost every monoamniotic pregnancy. * Cord compression: One twin may compress the other's umbilical cord, potentially stopping the flow of nutrients and blood and resulting in fetal death. *

Twin-to-twin transfusion syndrome Twin-to-twin transfusion syndrome (TTTS), also known as feto-fetal transfusion syndrome (FFTS), twin oligohydramnios-polyhydramnios sequence (TOPS) and stuck twin syndrome, is a complication of monochorionic multiple pregnancies (the most common ...

(TTTS): One twin receives the majority of the nourishment, causing the other twin to become undernourished. TTTS is much more difficult to diagnose in monoamniotic twins than diamniotic ones, since the standard method otherwise is to compare the fluid in the sacs. Rather, TTTS diagnosis in monoamniotic twins relies on comparing the physical development of the twins.

Diagnosis

Ultrasound is the only way to detect monoamniotic-monochorionic twins before birth. It can show the lack of a membrane between the twins after a couple of weeks' gestation, when the membrane would be visible if present. Further ultrasounds with high resolution

doppler imaging Inhomogeneous structures on stellar surfaces, i.e. temperature differences, chemical composition or magnetic fields, create characteristic distortions in the spectral lines due to the Doppler effect. These distortions will move across spectral li ...

and

non-stress test A nonstress test (NST) is a screening test used in pregnancy to assess fetal status by means of the fetal heart rate and its responsiveness. A cardiotocograph is used to monitor the fetal heart rate and presence or absence of uterine contractions. ...

s help to assess the situation and identify potential cord problems. There is a correlation between having a single

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far ...

and having a single amniotic sac. However, it is difficult to detect the number of yolk sacs, because the yolk sac disappears during

embryogenesis An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

. Cord entanglement and compression generally progress slowly, allowing parents and medical caregivers to make decisions carefully.

Treatment

Only a few treatments can give any improvements.

Sulindac Sulindac is a nonsteroidal anti-inflammatory drug (NSAID) of the arylalkanoic acid class that is marketed as Clinoril. Imbaral (not to be confused with mebaral) is another name for this drug. Its name is derived from sul(finyl)+ ind(ene)+ ac(etic ...

has been used experimentally in some monoamniotic twins, lowering the amount of amniotic fluid and thereby inhibiting fetal movement. This is believed to lower the risk of cord entanglement and compression. However, the potential side effects of the drug have been insufficiently investigated. Regular and aggressive fetal monitoring is recommended for cases of monoamniotic twins to look for cord entanglement beginning after viability. Many women enter

inpatient A patient is any recipient of health care services that are performed by healthcare professionals. The patient is most often ill or injured and in need of treatment by a physician, nurse, optometrist, dentist, veterinarian, or other health ca ...

care, with continuous monitoring, preferably in the care of a perinatologist, an obstetrician that specialises in high-risk pregnancies. However RCOG's guidelines cite Dias et al. in observing that cord entanglement is nearly always found in monoamniotic pregnancies and it is not clearly associated with poor outcomes, with most fetal deaths instead arising from

twin reversed arterial perfusion Twin reversed arterial perfusion sequence, also called TRAP sequence, TRAPS, or acardiac twinning, is a rare complication of monochorionic twin pregnancies. It is a severe variant of twin-to-twin transfusion syndrome (TTTS). In addition to the tw ...

or fetal anomaly. The clinical guidelines of ACOG and RCOG both recommend premature delivery by

cesarean section Caesarean section, also known as C-section or caesarean delivery, is the surgical procedure by which one or more babies are delivered through an incision in the mother's abdomen, often performed because vaginal delivery would put the baby or ...

between 32 and 34 weeks. A retrospective study in 2016 argued that there is evidence vaginal delivery can be equally safe and reduce complications for some monoamniotic twins but this finding has not been incorporated into clinical guidelines.

References

{{Pathology of pregnancy, childbirth and the puerperium Twin

Occurrence

Complications

Diagnosis

Treatment

See also

References