אמר רב חסדא טובלת ואוכלת עד ארבעים דאי לא מיעברא הא לא מיעברא ואי מיעברא עד ארבעים מיא בעלמא היא
Yevamot 69b
Rav Ḥisda said: She immerses and partakes of teruma only until forty days after her husband’s death, when there is still no reason for concern, as if she is not pregnant then she is not pregnant. And if she is pregnant, until forty days from conception the fetus is merely water. It is not yet considered a living being, and therefore it does not disqualify its mother from partaking of teruma.
Tzitz Eliezer 13:102
It is clear that in Jewish law an Israelite is not liable to capital punishment for feticide.... An Israelite woman was permitted to undergo a therapeutic abortion, even though her life was not at stake.... This permissive ruling applies even when there is no direct threat to the life of the mother, but merely a need to save her from great pain, which falls within the rubric of "great need." Now, is it possible to imagine a case in which there is more need, pain, and distress, than the present one, in which the mother is confronted by the [prospect of a] suffering child whose certain death is only a few years away and nothing can be done to save it?
(ו) הָאִשָּׁה שֶׁהִיא מַקְשָׁה לֵילֵד, מְחַתְּכִין אֶת הַוָּלָד בְּמֵעֶיהָ וּמוֹצִיאִין אוֹתוֹ אֵבָרִים אֵבָרִים, מִפְּנֵי שֶׁחַיֶּיהָ קוֹדְמִין לְחַיָּיו. יָצָא רֻבּוֹ, אֵין נוֹגְעִין בּוֹ, שֶׁאֵין דּוֹחִין נֶפֶשׁ מִפְּנֵי נָפֶשׁ:
(6) If a woman is having trouble giving birth, they cut up the child in her womb and brings it forth limb by limb, because her life comes before the life of [the child]. But if the greater part has come out, one may not touch it, for one may not set aside one person's life for that of another.
IM HM 2:69
Rabbi Moshe Finestein and Rabbi Auerbach both prohibit aborting a sick fetus, or testing for the disease in question, lest it encourage abortion.
כנעוריה פרט למעוברת. ואם תאמר תיפוק ליה מוזרע אין לה עיין עליה כי היכי דפסלינן בריש החולץ (יבמות לה:) חליצת מעוברת מאין לו עיין עליו וי"ל דהתם לא קאמר אלא עיין והמתן אם יהיה ולד של קיימא אבל לא מיפטרא מן היבום משום דכתיב אין לו וה"נ לא מיפסלא מן התרומה בהכי:
As a teenager except for the fetus. And if you say a leprosy, it is not necessary for her to look at it, because hichi Depselinan in the extractor brush (Yavmot La :) A fetal extract from which he has not looked at it and will fade not Kamer but look and wait if there will be a sustainable child but not get rid of the hibum because he has no dictation and " N. does not disqualify from the donation the most: