Brother, you are asking questions that dog many of God's people, complicated by the marketing of chemicals that seem to take the difficulty out of a vexing question. Christians want to do the right thing, but what IS the right thing?
As God's children we want to be Biblical. The scripture is silent in giving any direct commands concerning this issue, this of limiting one's family size. Some will quote David who says that happy is the man who has his quiver (or fliver? lol) full of them. But then how many arrows can a quiver hold? If it is like a fliver, it will vary to the size. So, we do know it will be several.
Others will point out the OT characters who had large families and we should emulate them in this regard. But they fail to realize that these children were not from ONE woman. Would they advocate polygamy to accomplish this goal? Not likely.
Then you have people quoting the scripture that children are a blessing from the LORD. This is true. However, I have had pregnant singles quote this to me inferring that if God wanted them to be pregnant she will be, or if she is it must be God's will. We would understand this to be a poor exegesis of scripture. This will now bring one to a basic concept of tempting God by challenging God to defy the laws of nature that HE has set in place to govern his creation. (Matthew 4:6)
It is a law of nature that when breeding occurs conception will likely be the result. This is true of all creatures that walk, swim, fly, slither or swim. This is God's law of nature at work and he rarely interferes in this process. Scriptures informs us of situations when he did, but this is not the normal course of life.
Scriptures also teach us that killing is sin. So God's children would understand that a fetus is a person as well as the one born and thus these lives should be protected and loved. With these principles in mind it would help one to discern how one should regard this issue.
First thing is you do not want to do is kill a child, by any means. So a wise person will investigate why and how contraceptives work. Does it prevent conception ALL the time or does it prevent the zygote from implanting in the uterine wall? This is why the pill is controversial. I have heard of godly physicians who will refuse to prescribe 'birth control' for this very reason. The pill literally is a 'birth control' procedure.
Where does this leave the couple who enjoy each other but believe they are not ready for a family or who believes they do not think they can handle 12 children? There are some wise persons who have studied nature and using the laws of nature advocate natural family planning. I am told that using this enhances a couples affection for each other because it is a joint venture.
Whatever you do, I would suggest you not kill a zygote either by pill use or IUD or any other way. (EDIT) Conception can be prevented/postponed by the proper use of nature....God is interested in you, your wife and any children that may grace your home. So, I would suggest you take Him into your consideration and allow Him to give you wisdom concerning this issue.
These are my thoughts, hope they can help.
ginnyrose