There are two ways of preventing miscarriages (spontaneous abortions):
- Not conceiving at all
- by abstaining from sexual intercourse (allowed)
- by contracepting (forbidden)
- Treating the underlying medical cause of recurrent miscarriage (allowed)
If #1 is intended, yet the husband and wife still engage in the marriage act, this is contraception, and the Church prohibits all forms of contraception (frustrating the procreative end of the marriage act). As Pope Pius XI said in his encyclical on Christian marriage Casti Connubii:
[A]ny use whatsoever of matrimony exercised in such a way that the act is deliberately frustrated in its natural power to generate life is an offense against the law of God and of nature.
Certainly, #1 could be intended by refraining from sexual relations, too, and the Church does not forbid this as long as both husband and wife agree to it.
The Catholic Church prohibits contraception in any form, regardless of other factors. If there is a serious reason for having marriage relations (e.g., to prevent a spouse from committing adultery), yet conceiving may be dangerous to her health (e.g., due to recurrent miscarriage), the couple can (and could even be obliged to) have marriage relations during the relatively more infertile period of her cycle. See, e.g., "Recourse to Infertile Periods" (§16) of Humanæ Vitæ.
Regarding #2, Catholics are certainly permitted to cure recurrent miscarriage, just as they are permitted and in some cases obliged, to cure any other disease. Failure to cure, if possible, a life-threatening disease could be a sin against the 5th Commandment. If curing the recurrent miscarriage produces some other undesired effect, like infertility, it might still be permissible to cure the recurrent miscarriage, as long as infertility is not intended; this is the principle of double-effect:
The principle that says it is morally allowable to perform an act that has at least two effects, one good and one bad. It may be used under the following conditions:
the act to be done must be good in itself or at least morally indifferent; by the act to be done is meant the deed itself taken independently of its consequences;
the good effect must not be obtained by means of the evil effect; the evil must be only an incidental by-product and not an actual factor in the accomplishment of the good;
the evil effect must not be intended for itself but only permitted; all bad will must be excluded form the act;
there must be a proportionately grave reason for permitting the evil effect. At least the good and evil effects should be nearly equivalent.
All four conditions must be fulfilled. If any one of them is not satisfied, the act is morally wrong.
An example of the lawful use of the double effect would be the commander of a submarine in wartime who torpedoes an armed merchant vessel of the enemy, although he foresees that several innocent children on board will be killed. All four required conditions are fulfilled:
he intends merely to lessen the power of the enemy by destroying an armed merchant ship. He does not wish to kill the innocent children;
his action of torpedoing the ship is not evil in itself;
the evil effect (the death of the children) is not the cause of the good
effect (the lessening of the enemy's strength);
there is sufficient
reason for permitting the evil effect to follow, and this reason is
administering a damaging blow to those who are unjustly attacking his
Usually recurrent miscarriage, like infertility, is a sign of some underlying medical issue that needs to be addressed. See ch. 12 "Recurrent Miscarriage" of Catholic OB/GYN Dr. Hilgers's The NaProTECHNOLOGY Revolution. Dr. Hilgers practices medicine in accordance with Catholic moral teaching.