Gemara

(Beraisa - R. Eliezer Ben Yakov): Each of Tzelafchad's daughters was at least 40 years old when she married.

Question: Rav Chisda taught that a woman that marries before age 20 will bear children until age 60. If she marries at 20, she will bear children until age 40. If she marries at 40, she will not bear children.

Answer: Since they were righteous, a miracle was done for them and they were able to conceive, like Yocheved.

Shabbos 32a: Men are judged when they pass over a bridge, or anything similar (any place of danger).

R. Yanai would check a ferry (for leaks) before going on it. He taught that one should not go in a place of danger, expecting a miracle. Perhaps a miracle will not be done for him; even if a miracle is done, it reduces his merits.

(R. Chanin): He learns from "Katonti mi'Kol ha'Chasadim..." (Yakov's merits were reduced through the miracles Hash-m did for him).

156a - Avraham Avinu: I see in my Itztagninus (the Chachmah of reading the stars) that I am not fitting to father a son!

Hash-m: I will change your Mazel.

Astrologers told R. Akiva that the day his daughter gets married, she will be killed by a snake. He was very worried. The day of her Chupah she did a great Mitzvah. Later, she stuck a pin in the wall. It struck a snake and killed it.

Chulin 9b (Rav Huna): We are more stringent about danger than about Isurim.

Berachos 60a (Mishnah): If one's wife is pregnant, it is a vain prayer to request that it will be a boy.

Question (Rav Yosef): Rav taught that Leah knew that twelve sons would be born. She reasoned that if she has a seventh son, Rachel will have less than one of the Shefachos! The fetus was (miraculously) switched to a girl.

Answer #1: We do not learn from miracles.

Answer #2: Leah prayed for this within 40 days of conception.

Kidushin 82a (Mishnah - R. Nehurai): I will not teach my son any trade, only Torah, for one enjoys its reward in this world, and the principal remains intact for the next world.

Yevamos 12b (Rav Bivi - Beraisa): Three women use a contraceptive wad to prevent pregnancy. A minor does so, lest she get pregnant and die. A pregnant woman does so lest the fetus get mashed. A nursing woman does so lest pregnancy force her to wean her baby, and it will die.

R. Meir says, a girl between 11 and 12 years uses a wad. If she is younger or older, she has relations normally.

Chachamim say, at any age she has relations normally. Shomayim will have mercy on her - "Hash-m guards the simple."

Rishonim

Rambam (Hilchos Rotze'ach 11:5): Chachamim forbade many things due to mortal danger. One who violates them and says 'I will endager myself. It does not harm others!', or 'I am not concerned for this' is lashed mid'Rabanan.

Rambam (12:6): One may not pass under a leaning wall or on a shaky bridge or enter a ruin. One may not stand in a place of all such dangers.

R. Bechaye (Bamidbar 16:21, brought in R. Akiva Eiger YD 116:5): Hash-m can kill and leave alive whomever He wants even if they are together - "Yipol mi'Tzidcha Elef..." Even so, he told everyone to separate from Korach, Dasan and Aviram, lest the bad air in Dever cling to them, like we find that Lot's wife looked at Sedom (and died). Alternatively, this is because when Midas ha'Din is ruling, it does not distinguish between Tzadikim and Resha'im.

Poskim

Rema (YD 116:5): One must beware of everything that leads to danger. Danger is more stringent that Isur. One must be more careful about Safek danger than Safek Isur. Therefore, Chachamim forbade to go in any dangerous place, e.g. under a leaning wall or alone at night. One may not rely on a miracle or endanger oneself.

Shulchan Aruch (CM 427:10): Anyone who transgresses these matters and says 'I will endager myself. What does is matter to others?', or 'I am not concerned for this' is lashed mid'Rabanan. One who is careful will be blessed.

Taz: The Medrash (Shir ha'Shirim 5:16) says 'not only does he (one who is careful) save himself from danger. He is even rewarded!'

Beis Yosef (YD 179 DH Kosav, citing Teshuvas Rashba (attributed to Ramban) 283: Astrology is not Nichush (superstition), which is forbidden. We find that Avraham heeded astrology, and R. Akiva was concerned for it. However, Hash-m sometimes does miracles for Tzadikim.

Birkei Yosef (OC 37:2): One opinion (cited in R. Yerucham, and Orchos Chayim citing Rav Yehudai Gaon) requires a clean body for one who relies on a miracle to wear Tefilin at a time of Nochri decrees against them. This is difficult, for Abaye and Rava said that he may not sleep or pass gas in them. They should require him to be a Tzadik like Elisha! The Rosh (Rosh Hashanah 1:5) explains that we discuss anyone who wears Tefilin.

Kitzur Shulchan Aruch (192:3): The Torah gave permission for a doctor to heal. Therefore, a Choleh may not rely on a miracle. He must conduct in the way of the world and seek a doctor to cure him. Many of the greatest Chasidim were cured through doctors. One who refrains from doing so does two evils to himself. Firstly, one may not rely on a miracle when there is danger. This causes his sins to be mentioned. Further, it is haughtiness to rely on his righteousness to be healed miraculously. He should call the most expert doctor, but his heart must be to Shamayim and request mercy from the faithful Doctor, and trust in Him.

Igros Moshe (OC 2:111 DH b'Davar): Taking an insurance policy does not show any lack of trust in Hash-m. It is like all business deals. One is not merely permitted, rather he must do business and work for his income. He may not say 'even if I do nothing, Hash-m will bring my income in some way.' How can one say that he has such merit? One may not rely on a miracle or pray for one, even if he is worthy of it. The Gemara asked why Leah prayed for a miracle, even though she was worthy of it and the Avos and Imahos were used to miracles. We answered that only she could pray for a miracle. Alternatively, even she did (and could) not! Therefore, one must engage in work and business, but he must know that what he gets is only from Hash-m, like was decreed on Rosh Hashanah. R. Nehurai said 'I will not teach his son any trade, only Torah.' He agrees that one may not rely on a miracle. He holds that most children cannot learn Torah properly if they also learn a trade then, and one is obligated to teach Torah to his son. He can rely on Hash-m to present to his son a job when his son is older, even if he did not learn a trade in his youth. The Rambam rules like R. Nehurai, for he does not obligate teaching a trade to one's son, yet the Rambam obligates one to get a job.

Igros Moshe (3 OC 69 DH uv'Davar): If one will be in mortal danger later on Shabbos, and Melachah will be required, may one do the Melachah now, before it is needed? We should be concerned lest he recover and the danger will pass. When there is danger, the Torah commands use doctors and cures and to transgress Shabbos and all Isurim and not to rely on miracles. Before there is danger, there is no obligation (to use doctors), and one may rely on a miracle that there will be no danger, even though doctors say that there will be. Therefore, one may not transgress Shabbos until there is danger. However, if later one will be unable to save him, surely one may not rely on miracles, and he must use all natural means now to save, even if he must transgress Shabbos and Isurim. The Amuk Davar (Korach 17:12, in the footnote) says that if Erev Shabbos is Yom Tov, and there will be danger on Shabbos, he should transgress Yom Tov to avoid Chilul Shabbos tomorrow, which is more stringent. This is difficult.

Igros Moshe (EH 1:63 DH v'Od): Birth is always a danger for which we are Mechalel Shabbos. It is a Mitzvah to have children; we rely on a miracle, for it is the way of the world. Chachamim rely on "Shomer Pesa'im Hash-m" also for the three women, for it is unreasonable that other women must rely on "Shomer Pesa'im", but the three women may not rely on it.

Minchas Yitzchak (9:136 DH v'Hinei): Divrei Yo'el concluded that if doctors say that it is dangerous for an adult to become pregnant, she has normal Bi'ah, and Hash-m will have mercy. He does not mean that she must do so. In Siman 124(3), he discussed a woman for whom it was dangerous to give birth. He said that since we rule like Chachamim, who forbid a was even for a minor who will surely die if she becomes pregnant, all the more so in this case, especially since doctors often lie and say through estimation that there is danger, and really there is no more danger than for a minor. Later, he retracted. Only for the three women, for whom it is the nature of the world that it is dangerous, Chacham say to rely on Hash-m's mercy. If a particular woman is sick, unlike nature, we may not rely on a miracle. However, we cannot bring a proof to here, for the husband need not agree to have Bi'ah with a wad. If he agrees to use methods to avoid relying on miracles, there is no Isur. Most Rishonim hold like this. However, if he did not yet fulfill Peru u'Rvu, he should not consent.