How is the Body of Christ like a fetus in the womb?
The Body of Christ is like a fetus in the womb because believers are added one at a time until the last person is placed in the Body. At that point it is complete, and God will deliver it from its earthly confines through the rapture, just as a fetus is delivered at birth.
Les explains that the analogy is one of development and delivery. The Body of Christ is not completed all at once; believers are coming into it one at a time. Just like a fetus in the mother's womb, one day the Body of Christ will have the last person in place. We don't know where it will be, but one day the last person is put into the Body, and it is complete. Then God will deliver it from its confines here on earth, and we will be raptured out, which becomes the 'until' of Romans 11:25. As soon as God has completed His work with the Gentile Body of Christ, He will turn to the Jew. The Jew will have to go through the seven years of tribulation before enjoying the blessings of Christ's return. As soon as the Church is gone, the Antichrist will appear, and the world will go into that seven years, which is predominantly the time God starts dealing with Israel. Then their blindness will fall away. In contrast, Luke 21 speaks of the unbelieving Gentiles, the unsaved world of Gentiledom, while Romans 11's fullness of the Gentiles refers to Church Age believers.
For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.