Shabbos 66b: A woman may go out on Shabbos wearing a retaining stone (to help her retain her pregnancy and not miscarry) or with another object that weighs the same as it – even if she has never miscarried in the past, and even if she is not yet pregnant.
שבת סו ע”ב: תנו רבנן: יוצאין באבן תקומה בשבת, משום רבי מאיר אמרו: אף במשקל אבן תקומה. ולא שהפילה ־ אלא שמא תפיל, ולא שעיברה ־ אלא שמא תתעבר ותפיל. ופירש הט”ז או”ח ש”ג כ”ד שטערי”ן שו”ס.
A woman was in the delivery room for hours but could not give birth. The husband called R’ Meir Scheinberg and asked him what to do. Rabbi Scheinberg asked, “Did she take off the ruby from around her neck?” She took it off, and immediately she was able to give birth.
Source: Rabbi Yehuda Aryeh Scheinberg