Matters Arising: Conjoined Twins

Rev Fr Nicholas Mary CSSR

Fr Nicholas answers topical questions in the light of moral theology and canon law.

What does the Church teach about conjoined twins? Do they have two souls and can they receive the sacraments? 

First of all, a definition. A conjoined — or ‘Siamese’ — twin is: 

‘One of a pair of twins who are physically joined and often share some organs. Fusion is typically along the trunk of the body or at the front, side, or back of the head. 

'In the case of symmetrical conjoined twins, the children usually have no birth anomalies except at the areas of fusion. In cases where each twin has enough tissue and organs for independent survival, they are often successfully separated by surgery. In the case of nonsymmetric conjoined twins, one is fairly well developed, but the other is severely underdeveloped, often tiny, and dependent on the larger twin for nutrition. ... 

'Siamese twins, the term formerly used for these children, originally referred to the conjoined twins Chang and Eng, who were born in 1811 to parents in Siam (now Thailand) and became widely known from their tours in the West. Chang and Eng were joined by a ligament from breastbone to navel.'1

Church teaching concerns itself with conjoined twins in three principal areas: their humanity, the conditions for their reception of the sacraments, and medico-moral questions regarding surgical interventions. 

Conjoined twins are sufficiently rare2  for there to be — to the best of this writer’s knowledge — no direct magisterial interventions in this matter. Rather, as for many questions in life, we are left to the guidance of sound moral theologians and the pastoral practice of the Church. We apply general principles, which are certain, to individual cases that are unclear, in order to arrive at courses of action which we can choose in good conscience. 

Fully human 

No treatment of the Catholic position of this subject can fail to mention these words of St Augustine in the City of God, addressed to an audience still affected by the views of the pagan world: 

'Whoever is anywhere born a man, that is, a rational, mortal animal, no matter what unusual appearance he presents in colour, movement, sound, nor how peculiar he is in some power, part, or quality of his nature, no Christian can doubt that he springs from that one protoplast [i.e. Adam]. We can distinguish the common human nature from that which is peculiar, and therefore wonderful. 

‘The same account which is given of monstrous births in individual cases can be given of monstrous races. For God, the Creator of all, knows where and when each thing ought to be, or to have been created, because He sees the similarities and diversities which can contribute to the beauty of the whole. But he who cannot see the whole is offended by the deformity of the part, because he is blind to that which balances it, and to which it belongs. We know that men are born with more than four fingers on their hands or toes on their feet: this is a smaller matter; but far from us be the folly of supposing that the Creator mistook the number of a man's fingers, though we cannot account for the difference. And so in cases where the divergence from the rule is greater. He whose works no man justly finds fault with, knows what He has done.

'Some years ago, quite within my own memory, a man was born in the East, double in his upper, but single in his lower half — having two heads, two chests, four hands, but one body and two feet like an ordinary man; and he lived so long that many had an opportunity of seeing him. But who could enumerate all the human births that have differed widely from their ascertained parents? As, therefore, no one will deny that these are all descended from that one man, so all the races which are reported to have diverged in bodily appearance from the usual course which nature generally or almost universally preserves, if they are embraced in that definition of man as rational and mortal animals, unquestionably trace their pedigree to that one first father of all.’3

Once the obvious humanity of the conjoined twins is accepted, then the Catholic finds himself in a very different position to that of primitive, superstitious cultures which viewed them as ominous monsters to be destroyed through infanticide (as did the ancient Romans and others), or at least to be condemned to a life of appearing in spectacles to gratify the curious. Likewise we are at odds with modern, ‘progressive’ ones which, out of a false compassion, seek to have them eliminated through abortion, or, like Dr Mengele in Auschwitz, to make them the subjects of hideous experiments in the name of science. The conjoined twin is simply and unequivocally a fellow human being created in the image and likeness of God, called to an eternal destiny, and redeemed by the Precious Blood of Our Lord Jesus Christ. The rights that he or she has by nature must be acknowledged and safeguarded, and the fruits of the Redemption applied through Baptism and the other sacraments.

The speculation of mediaeval Catholic theologians with regard to conjoined twins was motivated both by philosophical interest as well as by practical, pastoral concerns. Historian Irven Resnick notes how their work started from the assumption of twins’ humanity:

'First, unlike earlier chroniclers or theologians, Scholastics sought biological causes to explain the phenomenon of conjoined twins, especially in light of the Aristotelian principles of natural philosophy that were introduced to university culture in the first half of the 13th century. Second, they sought to understand the pastoral implications presented by conjoined twins that had been ignored by earlier authors. In particular, they raised questions in the quodlibetal literature about the baptism of conjoined twins — for example, whether one should baptize one twin or both — and they considered whether conjoined twins can contract a valid marriage. Positively, both discussions assume the humanity of conjoined twins, in contrast to popular practice that might instead kill such infants judged to be monstra.'4

Receiving the sacraments 

The results of their scientific speculations are today only interesting from an historical point of view, but their moral and pastoral conclusions found their way into canon law and the Roman Ritual, and are still observed. Thus Canon 748 of the 1917 Code prescribes that deformed or abnormal infants (Latin: monstra et ostenta) ‘should be baptized at least under condition; if there is doubt as to whether there is one or several humans, one should be baptized absolutely, the others under condition.’5  And Fr Halligan OP summarises the specifications of the traditional Ritual:

 'Anything born of woman that may be a living human being should be baptized at least conditionally: si es homo, or si es capax. In doubt whether an abnormal foetus or prodigy is single or multiple, one is baptized absolutely and the others conditionally: si es homo et nondum baptizatus, or si es capax.'6

These Latin formulae make the administration of the sacrament conditional: ‘If thou art human … If thou art capable [i.e. of receiving this sacrament] … If thou art human and not yet baptized …’ Fr Halligan notes that: 

‘The number of human beings actually or possibly present determines the procedure to follow. If there is certainly one human being present, no matter how poorly organized, it is baptized absolutely. If there are certainly two individuals conjoined in the one body, the head of each (or what reasonably seems to be the head) is baptized absolutely. If there are two heads and two hearts but one trunk, there are two individuals and both should be baptized absolutely; if the two heads are so united as to seem to be one head and two brains, they can be baptized in the plural (ego vos baptizo) and each heart baptized conditionally. If there are two heads and one heart or one body, there can be two persons and thus one head is baptized absolutely and the other conditionally. If there is one head and two hearts or two bodies, since it is not clear whether there are one or two persons, the head is baptized absolutely and each heart or body conditionally.'7

Once Baptism is received, the other sacraments may follow absolutely or conditionally according to the usual conditions of administration. In danger of death, Confirmation may be administered at once. Once the use of reason is attained, Penance and Holy Communion follow, and, when needed, Extreme Unction. The conjoined twins are, of course, members of the Church, and, as such enjoy 'the right of receiving from the clergy, according to the norm of ecclesiastical discipline, spiritual goods and especially that aid necessary for salvation.'8

What about Matrimony and Holy Orders?

As conjoined and unseparated twins live longer and longer, questions arise as to whether they can receive the sacraments of Matrimony and Holy Orders. With regard to the latter, suffice it to say that though the (in principle dispensable) legal irregularity by defect established by Canon 984 of the 1917 Code for 'those impaired in body who cannot safely because of the deformity, or decently because of the deformity, conduct ministry of the altar' has since been abolished, nonetheless, a decision by the ecclesiastical authorities would certainly be required in such an unusual and unprecedented case. The marriage of one or both twins is a situation rather more likely to occur. Though the conjoined twin and his or her prospective spouse should not enter the married state lightly due to the many and obvious difficulties that arise, nonetheless it does not seem that there is any fundamental impediment to the sacrament. One of the very few theologians to have addressed the matter, Eustache de Grandcourt (fl. 1300), has this to say:

'I say that they can contract marriage and with different [spouses], because they have everything by which marriage can be perfected, seeing that they have consent through which marriage is effected, and they have the instrument [of generation] by which it can be consummated, [and] since they have everything that is required for marriage, they will be able to contract and with different [spouses] since they have separate wills and separate [acts of] consent.'9

In the case of those twins who only possess one set of reproductive organs between them, however, questions of identity, adultery and incest are too complex to ignore, as Eustache notes. The civil law in many jurisdictions simply allows marriage wherever there is legal personhood, but the Church holds that 'antecedent and perpetual impotence, either on the part of the man or on the part of the woman, whether known or not, whether absolute or relative, impedes marriage by natural law itself.'10

Surgical intervention The moral principles that govern questions of surgical intervention are not particular to the case of conjoined twins, but rather those which hold good for all serious medical matters. Fr Edwin F Healy SJ writes:

‘If an operation is to be justified morally, it must be necessary, or at least proportionately useful, for the physical well-being of the patient. The graver the danger involved, the greater must be the surgeon’s reason for operating. Hence if surgery endangers the life of the patient, it may be performed for a grave reason only. A grave reason would be present if the operation were required to cure a serious ailment or to relieve severe chronic pain. Every major surgical operation must be considered potentially dangerous. In desperate cases a very dangerous operation is licit if there is a genuine (though slight) chance of success, for it is preferable to risk hastening the patient's death than to withhold this last possible remedy. The physician should as a rule acquaint the patient and his responsible relatives with the degree of danger involved in the proposed surgery and must obtain his consent before proceeding. This consent should be had explicitly when possible; but when this is not feasible, implicit permission will suffice.'11

Once the presence of conjoined twins has been detected by prenatal scanning, they may not be killed in the womb, of course. After birth, surgery to separate them may be undertaken if there is a well-founded chance of success, if the reason for permitting the danger is proportionate and if the parents consent. If the twins grow to adulthood conjoined, and surgery is still possible, then they can themselves decide on the matter. We should not automatically assume that conjoined twins desire this separation. Lori Schappell, one member of the longest-lived conjoined pair ever recorded (who died this year aged 62) repeated on many occasions, 'I don’t believe in separation. I think you are messing with God’s work.'12

All due care should be taken to baptise the infants before any surgery. The surgery may not deliberately aim at killing one twin to save the other, though the weaker, once separate, might naturally have a shorter life expectancy. Nor may a shared organ to which both have a right be taken from one that the other might lead a separate life if this will foreseeably directly cause the death of the first. Fr Healy writes in conclusion: 'Both have an equal right to the essential organ or part, which cannot be taken away from either one without inflicting a deathblow. There are circumstances in which it is not morally wrong to permit death by refraining from doing what might be done to prevent it, but there are no circumstances in which it is permissible to take positive measures to inflict death upon an innocent person.'13

  • 1Encyclopedia Britannica, online edition, article Conjoined Twin,
  • 2 Seminars in Perinatology, Vol. 42, 6, October 2018, p 319.
  • 3 Philip Schaff (ed.) Nicene and Post-Nicene Fathers, First Series, Vol. 2. (Christian Literature Publishing Co, Buffalo, NY, 1887) online at"> 
  • 4 Reflection on Individual Identity in Viator, Vol. 44/2, May 2013, p 347."> 
  • 5 in English translation with extensive scholarly apparatus (Ignatius Press, San Francisco, 2001).
  • 6Fr Nicholas Halligan OP, The Administration of the Sacraments (Mercier Press, Cork, 1963) p 47.
  • 7Ibid. fn 115 pp 75–76.
  • 8Canon 682 of the 1917 Code in Dr Peters’ translation.
  • 9Quoted in Irven Resnick, op. cit., pp 354 ff.
  • 10Canon 1068§1 of the 1917 Code, again in Dr Peters’ translation. This is similar to Canon 1084 §1 of the new code. This fundamental inability is of course different to impotence which develops during the course of a marriage, and again to infertility or sterility, none of which invalidate it.
  • 11Fr Edwin F Healy SJ, Medical Ethics (Loyola UP, Chicago, 1956), p 123.
  • 12
  • 13Op. cit. pp. 132–3.

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