The Rambam, of blessed memory, wrote in chapter 7: One who says to his fellow, "If your wife gives birth to a female, she is hereby betrothed to me with this" — he has said nothing; but if the wife of his fellow was pregnant and the fetus was recognizable — she is betrothed, and it appears to me that he must return and betroth her after she is born through her father, so that he may bring her into a betrothal in which there is no flaw. Thus far his words. And he holds that that which the sages, of blessed memory, said [62b] that where the fetus was recognizable she is betrothed, this is not only according to the one who holds that a person may transfer ownership of something that has not yet come into the world, but even according to us, who establish that a person cannot transfer ownership of something that has not yet come into the world, she is betrothed, for a recognizable fetus is like something that has come into the world. Many disagree with this, and therefore in the law it is a case of uncertain betrothal, and so our teacher, the Beit Yosef, ruled in section 8. And even though in Choshen Mishpat siman 209 regarding acquisition he did not distinguish in this matter, we have already explained in Choshen Mishpat there, section 6, that because of the severity of the prohibition of a married woman, he was concerned for the opinion of the Rambam, see there. However, the Rambam himself in chapter 22 of Mechirah wrote that one who grants to a fetus does not acquire, and he also did not distinguish between a recognizable fetus and one not recognizable. According to this, it appears that for the primary law he also holds that even with a recognizable fetus it is like something that has not yet come into the world, and only because of the severity of the prohibition was he concerned for this, and therefore he required him to betroth her when she is born. According to this, if he subsequently betrothed her sister, they both require a get, for it is an uncertain betrothal for both of them. And know, that even though the commentators of the Rambam and several of the poskim understood from his words that he holds that a recognizable fetus is not something that has not yet come into the world and she is fully betrothed, this is because they did not have before them his words in the Commentary on the Mishnah, for there he explained explicitly that it is a stringency in betrothal, and by the letter of the law even with a recognizable fetus it is something that has not yet come into the world. Our teacher Ovadia wrote similarly in his commentary on the Mishnayot in the name of the Rambam, that such was the version of the Mishnah before them, see there. [And one must further examine the Maharsha there who wrote that the version is an error, yet the Rambam and the Bartenura have that version, and so wrote the Tosafot Yom Tov; and according to this, they do not have the version in the Gemara comparing it to terumah, and his words in chapter 5 of Terumot are settled by this, see there, for there it says "when they are detached," and here "when she is born" is not applicable, for detaching is in one's hand as he wrote there; and one must further examine the Kesef Mishneh and the commentators of the Shulchan Aruch, and delve into it].
כתב הרמב"ם ז"ל בפ"ז: האומר לחבירו "אם ילדה אשתך נקבה הרי היא מקודשת לי בזה" – לא אמר כלום, ואם היתה אשת חבירו מעוברת והוכר העובר – ה"ז מקודשת, ויראה לי שצריך לחזור ולקדש אותה אחר שתלד ע"י אביה כדי שיכניס אותה בקדושין שאין בהן דופ י עכ"ל, וס"ל דזה שאמרו חז"ל [ס"ב:] דבהוכר עוברה מקודשת, אין זה למאן דס"ל אדם מקנה דבר שלא בא לעולם, אלא אפילו לדידן דקיי"ל אין אדם מקנה דבר שלא בא לעולם מקודשת, דהוכר עוברה הוה כבא לעולם, ורבים חולקים ע"ז ולכן לדינא הוה ספק קדושין, וכן פסק רבינו הב"י בסעי' ח'. ואע"ג דבחו"מ סי' ר"ט לעניין קניין לא חילק בזה, כבר בארנו בחו"מ שם סעי' ו' דמשום חומר האיסור דאשת איש חשש לדעת הרמב"ם, ע"ש. אמנם הרמב"ם בעצמו בפכ"ב ממכירה כתב דהמזכה לעובר לא קנה, ולא חילק ג"כ בין הוכר עובר ללא הוכר, ולפ"ז נראה דהעיקר לדינא ס"ל ג"כ דאף בהוכר עוברה הוה כדבר שלא בא לעולם ורק מפני חומר האיסור חשש לזה ולכן הצריך לקדשה כשתוולד, ולפ"ז אם קידש אח"כ אחותה צריכות שניהם גט דהוי ספק קדושין לשניהן. ודע, דאע"ג שמפרשי הרמב"ם וכמה מהפוסקים תפסו בדבריו דס"ל דהוכר עוברה לא הוה דבר שלא בא לעולם ומקודשת גמורה היא, זהו מפני שלא היה לפניהם דבריו בפירוש המשנה דשם ביאר מפורש דחומרא היא בקדושין ומדינא גם בהוכר עובר הוה דשלב"ל, וכ"כ רבינו עובדיה בפירושו למשניות משמו של הרמב"ם דכן היה לפניהם גירסת המשנה ע"ש, [וצ'"ע על המהרש"א שם שכתב שטעות הוא הגירסא, והרי הרמב"ם והרע"ב גורסים אותה, וכ"כ התוי"ט, ולפ"ז לא גרסי בגמ' הדמיון לתרומה, וא"ש בזה דבריו בפ"ה מתרומות ע"ש, דהתם אמר לכשיתלשו וכאן לא שייך לכשתלד דתלישה בידו הוא כמ"ש שם, וצ"ע על הכ"מ ומפרשי השו"ע, ודו"ק].