Locked twins is a rare complication of

multiple pregnancy A multiple birth is the culmination of a multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such births ...

where two fetuses become interlocked during

presentation A presentation conveys information from a speaker to an audience. Presentations are typically demonstrations, introduction, lecture, or speech meant to inform, persuade, inspire, motivate, build goodwill, or present a new idea/product. Presenta ...

before birth. It occurs in roughly 1 in 1,000 twin deliveries and 1 in 90,000 deliveries overall. Most often, locked twins are delivered via

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

, given that the condition has been diagnosed early enough. The fetal mortality rate is high for the twin that presents first, with over 50% being

stillborn Stillbirth is typically defined as fetal death at or after 20 or 28 weeks of pregnancy, depending on the source. It results in a baby born without signs of life. A stillbirth can often result in the feeling of guilt or grief in the mother. T ...

Description

There are two types of locked twins: breech/vertex and vertex/vertex. In breech/vertex presentations, which are much more common, the first twin is in the

breech position Breech may refer to: * Breech (firearms), the opening at the rear of a gun barrel where the cartridge is inserted in a breech-loading weapon * breech, the lower part of a pulley block * breech, the penetration of a boiler where exhaust gases leave ...

, presenting feet-first, and the second is in the

cephalic A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may no ...

(vertex) position, presenting in the normal head-first manner. In these cases, the chin of the first twin locks behind the chin of the second twin while in the

uterus The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...

birth canal In mammals and other animals, the vagina (: vaginas or vaginae) is the elastic, muscular reproductive organ of the female genital tract. In humans, it extends from the vulval vestibule to the cervix (neck of the uterus). The vaginal int ...

, preventing

vaginal delivery In mammals and other animals, the vagina (: vaginas or vaginae) is the elastic, muscular sex organ, reproductive organ of the female genital tract. In humans, it extends from the vulval vestibule to the cervix (neck of the uterus). The #Vag ...

. In vertex/vertex presentations, where both twins are positioned for head-first delivery, the two heads become locked at the

pelvic brim The pelvic brim is the edge of the pelvic inlet. It is an approximately butterfly-shaped line passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis. Structure The pelvic ...

, preventing either fetus from passing through the

pelvic inlet The pelvic inlet or superior aperture of the pelvis is a planar surface which defines the boundary between the pelvic cavity and the abdominal cavity (or, according to some authors, between two parts of the pelvic cavity, called lesser pelvis an ...

in a vaginal delivery. Contributing factors to the interlocking of twins include uterine

hypertonicity In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane ...

, small fetal size, and reduced levels of

amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products betwee ...

following rupture of the membranes. It is more likely to occur in women with large pelvises, young

primigravida In biology and medicine, gravidity and parity are the number of times a female has been pregnant (gravidity) and carried the pregnancies to a viable gestational age (parity). These two terms are usually coupled, sometimes with additional terms, t ...

e (young women in their first pregnancy), and pregnancies with

monoamniotic twins Monoamniotic twins are identical or semi-identical twins that share the same amniotic sac within their mother's uterus. Monoamniotic twins are always monochorionic and are usually termed Monoamniotic-Monochorionic ("MoMo" or "Mono Mono") twins. ...

Management

If locked twins are diagnosed in early delivery, before either twin is born, both fetuses can be delivered by

. If one fetus has been partially born, attempts can be made to disimpact the twins manually, such as by the Zavanelli maneuver, with a view to performing an assisted delivery with

ventouse Vacuum extraction (VE), also known as ventouse, is a method to assist delivery of a baby using a vacuum device. It is used in the second stage of labor if it has not progressed adequately. It may be an alternative to a forceps delivery and cae ...

forceps Forceps (: forceps or considered a plural noun without a singular, often a pair of forceps; the Latin plural ''forcipes'' is no longer recorded in most dictionaries) are a handheld, hinged instrument used for grasping and holding objects. Forcep ...

. If the diagnosis is made only after the first locked twin has died in the birth canal, or if it is not expected to survive, the first twin may be

decapitated Decapitation is the total separation of the head from the body. Such an injury is invariably fatal to humans and all vertebrate animals, since it deprives the brain of oxygenated blood by way of severing through the jugular vein and common ...

and its head pushed up to allow safe delivery of the second twin. At least one case has been reported where

hexoprenaline Hexoprenaline is a selective β2 adrenergic receptor agonist used in the treatment of asthma. Hexoprenaline is also used in some countries (such as Russia and Switzerland) as a tocolytic agent (i.e., labor suppressant), with the most common trad ...

was used to relax the uterine

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

to thereby temporarily inhibit labour and disimpact the fetal heads.

Outcomes

Since locked twins are often diagnosed in the late stages of delivery, it is often too late to intervene to save the life of the first twin and thus there is a high rate of

stillbirth Stillbirth is typically defined as fetus, fetal death at or after 20 or 28 weeks of pregnancy, depending on the source. It results in a baby born without vital signs, signs of life. A stillbirth can often result in the feeling of guilt (emotio ...

, estimated to be over 50%.

References

External links

{{Pathology of pregnancy, childbirth and the puerperium , state=expanded Complications of labour and delivery Twin Stillbirth