Locked twins is a rare complication of multiple pregnancy where two fetuses become interlocked during presentation 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 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 mo ...

, 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.

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 A breech birth is when a baby is born bottom first instead of head first, as is normal. Around 3–5% of pregnant women at term (37–40 weeks pregnant) have a breech baby. Due to their higher than average rate of possible complications for the ...

, 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 ...

(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'', 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 ...

birth canal In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen. ...

, preventing

vaginal delivery A vaginal delivery is the birth of offspring in mammals (baby, babies in humans) through the vagina (also called the "birth canal"). It is the most common method of childbirth worldwide. It is considered the preferred method of delivery, with l ...

. 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 Mickey Mouse head-shaped line passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis. Structure The pel ...

, 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 and ...

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 human medicine, gravidity and parity are the number of times a woman is or has been pregnant (gravidity) and carried the pregnancies to a viable gestational age (parity). These terms are usually coupled, sometimes with additional te ...

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 The Zavanelli maneuver is an obstetric maneuver that involves pushing back the delivered fetal head into the birth canal in anticipation of performing a cesarean section in cases of shoulder dystocia. The Zavanelli maneuver is performed only af ...

, 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 ...

or forceps. 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 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 an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...

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 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 result in the feeling of guilt or grief in the mother. The term ...

, estimated to be over 50%.

References

External links

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