All my days are ordained before I was born?

Psalm 139:16 says:

"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."

For me, this is proof that DNA was created by God & not by a series of 'FORTUNATE ACCIDENTS' that took place over eons of years.

This is my own take on this.

Discovering Your Destiny

When it comes to discovering your destiny, it’s very important to understand that it was created for you before you were born. Jeremiah 1:5 says,

“I (God) knew you before I formed you in your mother womb.

This is important because you are not here by accident. You were sent to this planet with an assignment and a destiny to fulfill.

God created you in heaven, thought about what he wanted you to do and then created a plan for you to do it and then sent you to this planet to fulfill your destiny. But finding your destiny for most is a very difficult thing. From the moment you are born, forces in this world attempt to create terrible circumstances, roadblocks and detours to prevent you from finding your destiny. This leaves most people feeling hopeless, depressed and confused. The result is they aimlessly wander through life trying to find what’s best for them hoping they bump into their destiny.

In heaven there is a book with your name on it that contains every detail of your life. This book is God’s plan for your life and destiny. It is filled with instructions, blueprints, ideas, concepts, as well as people you are supposed to connect with on your path in life.

The greatest thing about God’ plan for you is that it contains no failures. You may fail, you may fall, you may get off track and make many mistakes but God’s plan for you never changes. His plan is fail-proof and was written for you to prosper. God said Himself, (Jeremiah 29:11)

“For I know the plans I have for you,” declares the LORD, “plans to prosper you and not to harm you, plans to give you hope and a future.”

Your job in life is to find your book, read it, choose your destiny and apply it to your life. This will keep you from a lifestyle of randomness, and instead, you’ll experience a fulfilled life with the satisfaction of being in purpose.

This can only be found in Christ the one who created your book.

To answer the question "How should we interpret this verse?":

This question opens the rather large kettle of fish of soteriology when it comes to the suggestion that our entire lives are predestined before we are born. As you may have seen from other answers, some - such as Calvinists - happily affirm that yes, God predestines and controls every aspect of our lives, while Arminians might say the verse simply refers to God's foreknowledge. There is no one view that people hold. I think it is unlikely that the verse is referring (solely) to embryology though - the more literal translators tend to indicate it refers to our future days:

"Mine unformed substance Thine eyes saw, And on Thy book all of them are written, The days they were formed..." (YLT)

"Your eyes saw my unformed substance; in your book were written, every one of them, the days that were formed for me, when as yet there was none of them" (ESV)

And the NIV as you saw. However, as a previous answer stated, there may be room for translation error/flexibility.

Personally, I believe the verse does refer to predestination, but that doesn't mean we don't have a choice! I go more in-depth explaining this in my answer to this question, but essentially God can plan out every single one of our days without causing us to do anything or being responsible for our actions - in other words, without removing our free will. I firmly believe whether or not you go to Heaven or Hell depends on your choice - you are not a puppet.

TL;DR: The majority view is that the verse refers to our future days, some say it means God predestines and controls everything, others that he just knows everything, I take a middle ground.

Reformed (i.e. Calvinist) denominations such as my denomination (Presbyterian Church in America) indeed use this verse as support for predestination, especially as it pertains to God's eternal plan (referenced here).

Few Arminians use this verse as a proof text against predestination, although I did find one example:

In the bible it says (Psalm 139:16) that God knows all our days in advance, yet we live one at a time. God has a foreknowledge of the future and knows it to the detail.

...

Ask yourself. If [predestination is true], why would we need to live one day at a time?

Additionally, this is a popular verse for opponents of Abortion (as a whole from verses 13-16). Here is an excerpt from a sermon on this subject:

But probably the best known passage of Scripture on this subject is Psalm 139:13-16. A couple of Sabbaths ago I preached on God’s omnipresence, the doctrine that God is always present in every event and God always is with us in every place and circumstance. For that earlier sermon, I took as my text Psalm 139:7-12. In the verses before us today, in verses 13-16 of that same Psalm, the Psalmist continues to describe God’s presence with us, even in the womb. In v13, speaking of God, the Psalmist says, “It was you who formed my inward parts; you knit me together in my mother’s womb.” In verses 15 and 16 the Psalmist praises God saying, “My frame was not hidden from you, when I was being made in secret, intricately woven in the depths of the earth. Your eyes beheld my unformed substance. In your book were written all the days that were formed for me, when none of them as yet existed.” The Psalmist says that when I was conceived, that was the first day of my life God was there with me. When my heart began to beat, God was there with me. When my fingers and toes and muscle and brain formed, God was there with me.

First off Psalms are filled with hyperbolic phrases.

The translation for this passage is much debated and commented on. I would put most of the latter parts of comments in the speculation category. They are filled with speculative arguments and implications like God planning out our lives in detail. Then the jump comes from theses speculations to things like predestination.

The first part of comments in commentaries is about the meaning of the Hebrew- an embryo and unformed mass being developed and the ordained days of this being written in a book. Nothing is said about a specific plan laid out as to who he will marry or what he will eat or eve what he will do. It is more of a general wow! that what is unrecognizable is known to God. We could run with this and say well if he knew it and wrote that down and he knows everything, doesn't he know what will happen each day including our being saved what we will eat etc.? Maybe maybe not. Even if that is true does this verse say that? I don't believe so based on the literal translations? Yes plural because it is a kind of difficult passage to translate.

Then we could jump to the next implication that if He knows our formed body before it is formed even though there is no evidence that it will be a body, couldn't we imagine He knows all the other details of what we think, feel, decisions we will make? Its logical based on the whole Psalm 139 passage BUT without reading into this verse is not saying those things.

Most believe David wrote the Psalm so maybe this level of "ordained days" are only for him or specially called great men of God and does not apply to all men. Are there comparative passages that imply this is the Normal Practice and belief and should therefore be considered teachable doctrine?

It is a person expressing adoration to an omniscient God. Nothing is explicitly mention about plans or predestination and no application is given except that it should elicit praise.

If you start with the attributes of God, that He is omnipotent, omnipresent and omniscient, then it follows that he is sovereign over all things - the universe, the earth and individuals. It is often hard for humans to give up their autonomy and acknowledge that God is in charge. Until we do, we worship a god of our own creation. These verses definitely support God's sovereignty from before I was born to all the days of my life. Beautiful! -- and comforting!

Foreordination is not the same as predestination.

Ordination in the Bible typically refers to being set apart to the office in the Priesthood, for example, the members of the house of Aaron were ordained to take care of the tabernacle during the exodus and later the temple.

A great example of someone who did not follow through with his ordination is Kind David. Even though he was ordained to great things he committed great sin and lost his opportunity.

When the bible refers to foreordination it means that God has identified the potential of certain individuals and ordained them to do great things before they were born. That said, someone does not have to follow through with their potential because we were each given what's probably the most difficult gift for any father to give: the freedom to choose.