A woman who was twice widowed is referred to as a "katlanit", a "deadly" woman. The Talmud rules that a katlanit should not marry a third time, based on the presumption that when tragedy has struck twice, it is likely to strike a third time in identical circumstances. This is true only if the two husbands that died were from one marriage after the other. A woman is not rendered a katlanit, however, if there were other marriages between the death of one husband and the other.
It was for this reason that Yehuda did not allow his daughter-in-law Tamar to marry his son Shela after her first two husbands (also his sons) had died. From this precedent some authorities maintain that the concept of katlanit is actually one which derives from the Torah and is not merely a custom or rabbinical enactment. There is some difficulty in labeling Tamar a katlanit, however, considering that the Torah itself states that her first two husbands, Er and Onan, died as a result of their own sins.
There are two opinions in the Talmud as to what exactly causes a woman's husband to die. According to one opinion there is something mystical in their intimate relationship which causes her husband's death. According to the other opinion it is a result of a woman's natural bad luck which brings upon such misfortunes. It is generally the first opinion which is considered to be more authoritative for all practical purposes. For example, a woman whose first two husbands died after they were formally married but before the marriage was actually consummated is not deemed a katlanit.
A woman is only considered to be a katlanit if her two deceased husbands both died of natural causes. If, however, one of the husbands was murdered or died due to a work accident, the woman is not deemed a katlanit. Similarly, if one of the husbands died at an advanced old age, the death of that husband will not render a woman a katlanit. So too, a woman who marries a man who is known to have a pre-existing health condition and later dies as a result of it will not be considered a katlanit, either. It is interesting to note that such health conditions include obesity which is known to trigger subsequent health complications. Some authorities are of the opinion that a woman who is independently wealthy is not subject to any katlanit considerations or consequences.
It is interesting to note that historically, authorities were never too quick to enforce any katlanit restrictions and never took any steps to block such marriages. One of the justifications for this approach was the fact that the chance of a third or fourth husband mysteriously dying is so unlikely that opposing such a marriage is not warranted. It is also noted that finding a mate is not an easy task, and as such, we should take an encouraging disposition towards those who have decided to get married notwithstanding any katlanit concerns. Indeed, although one may want to think twice before marrying a katlanit there is certainly no obligation for such couples to divorce.
Even the Rambam, who is of the opinion that one should not marry a katlanit writes that "there is no prohibition involved at all" if one chooses to do so. It might even be that the Rambam only counsels those who believe in superstitions not to marry a katlanit while all others need not worry at all. It is also argued that a young woman who is declared a katlanit and as a result is disqualified from marrying or is severely limited in her choice of potential marriage partners is likely adopt a lifestyle incompatible with halacha, which is certainly far worse than any katlanit considerations.
Further grounds for leniency is the principle of "Shomer Petaim Hashem" meaning, "God guards the simple", which a number of authorities invoke in order to permit one to marry a katlanit. According to this principle, something that is considered to be a mainstream or routine activity is permitted even though it might include dangerous elements. This is true as long as the risks involved are statistically negligible. Death-by-marriage is certainly one of those things whose risks are considered to be negligible. As such, many authorities permit one to marry a katlanit based on this reasoning.
Some authorities have extended the idea of a katlanit to include a woman who has been divorced twice, though the halacha is not like this view. There is no equivalent concept of katlanit with regards to a man. This is based on the belief that the reason for a woman becoming a katlanit was due to some Divine decree that she remain poor without a husband to provide her with her livelihood. A man, however, is generally not dependant upon his wife for support and therefore the death of one or more wives could not be the result of such a decree. Also related to the concept of katlanit is that of a person who lost two sons as a result of their brit mila. In such a terrible situation any additional boys that are born are not to be circumcised.
 Yevamot 64b.
 Bereishit 38:11.
 Beit Shlomo 18; Ketav Sofer 13.
 Bereishit 38:7,10.
 Chavot Yair 197; Otzar Haposkim 9:64.
 Beit Shmuel, EH 9:6; Chatam Sofer, EH 23; Igrot Moshe, EH 4:42; Yabia Omer, EH 3:5, 8:16.
 Teshuva M'ahava 1:41.
 Az Nidberu 13:58.
 Pitchei Teshuva 9:2; Noda B'yehuda 9.
 Terumat Hadeshen 211.
 Rambam, Hilchot Issurei Biah 21:31; EH 9:1.
 Kesef Mishna, Hilchot Issurei Biah 21:31;Teshuvot Harambam (Pe'er Hador) 146.
 Terumat Hadeshen 211.
 Tehillim 116:6.
 Shabbat 129b; Yevamot 12b; Nidda 31b; Avoda Zara 30b.
 Maharanach 36, Mishne Halachot 4:190; Yabia Omer, EH 8:16.
 Rashi, Yevamot 26a; Rema, EH 9:1; Rivash 333; Minchat Yitzchak 7:108.
 Teshuvot Harosh 53; EH 9:2.
 YD 263:2. See also Rashi to Shemot 12:48.