If you have a Bible that shows several words in verse 16 that are italicized, those were added to the original Hebrew text for clarity, which will be more evident below. The days were linked to the subject of the formation of the embryo, God seeing the whole process of development before there was an embryo.
The Hebrew in English words would read "embryo-of-me they-saw eyes-of-you and-on scroll-of-you all-of-them they-are-being-written days they-were-formed and-not one in-them"
Context is had by looking around that. Here's KJV again, skipping over a lot of Hebrew transliterations:
Psalm 139:14-16 (KJV)
[SUP]14 [/SUP] I will praise thee; for I am fearfully and wonderfully made: marvellous are thy works; and that my soul knoweth right well.
[SUP]15 [/SUP] My substance was not hid from thee, when I was made in secret, and curiously wrought in the lowest parts of the earth.
[SUP]16 [/SUP] Thine eyes did see my substance, yet being unperfect; and in thy book all my members were written, which in continuance were fashioned, when as yet there was none of them.
In verse 14 and 15 "substance" has the same meaning of "embryo of me", described as having parts or members in English, "members" in italics for clarity. In verse 15 "wrought" is from Hebrew meaning "I was embroidered".
I must then opine that the "days" have to do with the period of embryo development until the perfect body was formed and finished, complete with working mind, and his or her spirit.
Those days are very vital ones, we would all agree. But I don't find backup scriptures saying that verse means God has written out all the days of our lives. He does have a plan for each of us. If we submit to God, avoid evil, resist the Devil, the Devil will flee, leaving us to find God's plan for each of us and execute it. Psalm 34:14-16 (KJV)
[SUP]14 [/SUP] Depart from evil, and do good; seek peace, and pursue it.
[SUP]15 [/SUP] The eyes of the LORD are upon the righteous, and his ears are open unto their cry.
[SUP]16 [/SUP] The face of the LORD is against them that do evil, to cut off the remembrance of them from the earth.
There is no limit of days for that promise.
My wife and I love the fact that our beginning days of formation were so dearly watched by God, and thank God for a healthy start. We pray those verses as part of prayers for healing almost everyday over someone. That and several more scriptures build faith, and results follow. That helps the sick to turn their focus from their ailments to that wonderful assurance of how much God cares for every person, brought to term and birthed, or not.
This is partly why many Christians are rightfully so opposed to abortion, seeing how much God values an unborn child, an infant, a toddler, a teen, adult, senior adult. Our body is made to last a long time, deserving the respect God has for it. Jesus died so ours would not require many stripes.
We include the conclusion of the thoughts of God given by inspiration to David.
Psalm 139:17-18 (KJV)
[SUP]17 [/SUP] How precious also are thy thoughts unto me, O God! how great is the sum of them!
[SUP]18 [/SUP] If I should count them, they are more in number than the sand: when I awake, I am still with thee.
Selah!