The number 40 is symbolic. When the flood came it rained 40 days and 40 nights. The people of Israel wandered for 40 years in the desert before coming to the Promised Land. Jesus fasted and prayed in the wilderness for 40 days after His baptism by John in the Jordan to prepare for His ministry. This 40 day waiting to come to Church on the part of the mother and child need not, in my opinion, be taken literally. I believe it underlines the perspective the Orthodox (and all orthodox) Christians have that each person who has been called and has accepted to follow Christ is an "alter Christus", another Christ. That is we must allow the Lord's character to grow within us, so that we mirror Him in our lives, each in our own unique and personal way.

The practical reason for this abstaining from gathering for worship is the fragile and vulnerable state of the mother and the newborn after the momentous entrance into the world of this new "alter Christus". Nowadays especially in the west this is not as great a concern as it once was. It is probably more advantageous for the mother to come to worship and receive the Holy Mysteries as soon as possible so she can be healed, encouraged, inspired, directed in her special new ministry as mother. In that case the child should be churched as soon as she and he/she arrive in Church. And once the child is baptized it will receive Communion.

The perspective of the Pastor is very important here and I would consult with him as to what I have advised. If he feels strongly that the 40-day abstinence ought to be observed, then there may be a greater value in patient obedience at this point than in the immediate re-entry into public worship. By the way, obedience does not always mean that you agree with the one you obey, nor that he or she are right in what they require of you - but there are priorities in life and it may not be wise to insist on one's own perspective in this matter. One ought to choose one's battles carefully. There are so many occasions to disagree.

I hope this is helpful. I am not sure you could say it is "the teaching of the Church". We Orthodox do not have a unanimous, all-embracing magisterium that gives infallible pronouncements. We seek the wisdom of men and women of the Church especially that of Pastors - Bishops (the foremost teachers of the Church), Priests, Deacons. We carefully examine our Tradition and our traditions and do the best we can, asking God for mercy and for help. We keep the faith. At any rate, the above is what makes sense to me and I have been a Pastor for thirty-plus years. (That does not necessarily make me right, by the way).