Conception (fertilization) is the only answer.
People try to argue with this, but in the scientific world they are spending billions looking for "life" on other planets, comets, and so on. You can bet everything that you have, that if they found a fertilized egg on a comet or meteor, they would spend billions protecting it and nurturing that life. The headlines would proclaim "Life finally found in outer space!" It would be the arguably the greatest discovery of the human race...
Life begins in that precious moment when the feminine beauty of an egg and the masculine strength of a sperm unite. It's then that the cells begins dividing. Either on it's own can never produce life, but once they do beat all odds and unite, a precious life has begun.
If you are pregnant, I know it's natural to be scared. You are much braver than you know. Use that bravery to protect the vibrant life growing inside you. It's no "parasite". "Jane Roe" of Roe vs. Wade status, helped usher in abortion. About 10 or 15 years after this, while she was working in an abortion clinic, a little beautiful girl who was the daughter of the Christian Pro-lifers next door in Dallas, TX, won her heart and led her to Jesus. This lady who championed ending life in their mother's womb wrote a book, called, Won by Love, where she details her story, how the abortion movement used her for their poster child and didn't even let her get an abortion during the trial, because it would get more sympathy for her to be pregnant during the infamous Roe vs. Wade. So she carried the baby through and gave her up for adoption. Anyway, she tells of all the vats of tiny little human limbs sucked out of their mothers own womb. Although she worked at the clinic, she got too close to the procedure one day and saw that what came out was a little baby human....that and meeting this innocent precious little girl who just loved her even though they were supposed to "enemies" (prolifers and prochoicers), melted her broken and hurt heart. Anyway, please order the book of ebay or amazon. It's at least worth one read from the woman who actually was Jane Roe and who years later and until her death tried to undo what she had accomplished that terrible day in the courts. Or at least find a copy of the new movie of a lady who worked with planned parenthood for years, Abby Johnson. They just made a movie, it was in the movies in March, about her story and about your very important question. It's called "Unplanned".
Lastly, I know you may be very scared if you are pregnant, but consider this... if the prolifers are right, then you carry a very special creation of God entrusted to your care...as you were entrusted to your mother's care. I don't know what kind of mother you had, but what kind of mother do you want to be? If the prolifers are right, then a great and terrible wrong or sin will be done that cannot be undone, and if they are wrong about their position, then the worst is that a new baby will be born into the world, and it can at least be given up for adoption if you don't desire to be it's mother....If the prochoicers are right, then you simply get a convenience to the baby growing inside, but if they are wrong, a murder is committed...one of the greatest human wrongs or sins against God and against another human being.
You are asking questions and that is a good sign. Please find the fierce bravery that is in every mothers heart and fight for this life within you. If the simplest formation of a sperm and egg where found on a meteor, it would be groundbreaking PROOF that there is life in space, why would it not be life if it's a human child inside the safety of his or her mother's womb? It's just nonsense, we all know what life is in our hearts, no matter what lies we are told...
Jesus loves you very much and he loves all babies (calling a baby a fetus is like where they call people whom they want assassinated "subjects" or "targets" to dehumanize what they are doing...killing another human life. You are precious and life is precious. Jesus died on the cross so that everyone could find eternal life in Him and salvation through asking him to forgive their sins. Life is precious. As soon as a sperm and egg unite, that is a baby. It may be the smallest a baby can be, but it's a baby anyway, just like when a mother kangaroo (a marsupial) gives birth to this tiny tiny little thing that finds it's way into her pouch. It doesn't look anything like a kangaroo, but regardless it still is a tiny pink hairless little joey. Yes babies look quiet different from adult humans, especially at conception, and even after birth they look more like the person that they'll grow up to be, but they're so small and shriveled and their legs and seem all in the wrong position... but they are still baby humans...even as they progress as chldren to adulthood, we all look different as different stages...and even as we grow very old and once again become bent and shriveled. Humans are always changing, it's just it normally happens so slowly that we don't notice and can also see it with our own eyes, but even before we can see him or her with our own eyes, it's still a baby human going through the same progression that we all go through. Each age, even conception is very precious , and life deserves to be loved and protected, just as you yourself deserve love.