Life starts at conception
When an egg is fertilized there is a possibility it may become a human being, however this does not always happen, because for example these eggs may be disposed of through a woman's menstrual cycle. Most miscarriages are due to a chromosomal mixup.
"As an infertility specialist, I witness human fertilization in the laboratory every day. The human egg is a single living cell and it becomes a one-cell embryo if it successfully combines with a live sperm. No new life is formed — the egg and the sperm were already alive — and fertilization is not instantaneous. Nearly 48 hours pass from the time sperm first bind to the outside of the zona pellucida, the human eggshell, until the first cell division of the fertilized egg. The two newly formed cells then have the potential to give rise to a human being, but only if they are appropriately nurtured so that they continue to divide and then successfully implant in the uterus." - https://www.timesunion.com/opinion/article/Why-life-doesn-t-begin-at-conception-12320582.php
The debate around where life beings is entirely a religious one and has nothing to do with science.
Some good sources
https://jme.bmj.com/content/34/10/727