John 3:5: "Of water" refers to physical birth from amniotic sac --- who first proposed that theory? Amen, amen, I say to you — if a man is not born of water and Spirit,
  he cannot enter the kingdom of God.


Some say that the phrase "of water" refers to physical birth. Thus, one must be born in order to be born again, a logical progression of sorts. But, who first proposed this interpretation of John 3:5?

Disclaimer: This is not my belief. I'm asking who first proposed it because I'm writing a paper on the topic. Thank you.
Short Answer: I was unable to find anything in support of this view prior to 1989.

D. A. Carson's commentary on John (generally considered to be the best available commentary on this book of the Bible) explains the following: (See p. 191)


  Noting that v. 6 describes two births, one from flesh to flesh and the other from Spirit to Spirit, some interpreters propose that 'born of water and the Spirit' similarly refers to two births, one natural and the other supernatural. . . . To support this view, 'water' has been understood to refer to the amniotic fluid that breaks from the womb shortly before childbirth, or to stand metaphorically for semen. But there are no ancient sources that picture natural birth as 'from water', and the few that use 'drops' to stand for semen are rare and late.


Thus, it would seem that there are no ancient sources to cite for this view. This is confirmed by The NIV Application Commentary: (see p. 115)


  The chief problem here is that this culture did not refer to natural birth as birth "by water" (although we may do so today, thinking of water as either amniotic fluid or semen.)


For a defense of the natural birth view, Carson points us to two sources:


Ben Witherington III, New Testament Studies 35, (1989), 155-160
L. Morris, Jesus is the Christ: Studies in the Theology of John, (IVP, 1989), 150-151


Andreas Kostenberger's commentary on John (which is ranked #2 on the aforementioned site) suggests two additional sources for a defense of the physical birth view:


B. Witherington, John's Wisdom, (Louisville: Westminster John Knox., 1995), 97
J. C. Laney, Moody Gospel Commentary: John, (Chicago: Moody, 1992), 78




Here is some additional information that I came across which may be helpful:

The New American Commentary also supports Carson's claim: (see p. 173-174)


  These two words also should not be bifurcated as in some inadequate folk interpretations of the text where water is equated with the water of natural birth (either that of the sack in which the baby floats or the male fluid of the sex act).


Two sources are suggested for further research (although I am not sure if they are defenses or refutations):


Beasley-Murray, John, 48
H. Odeberg, The Forth Gospel Interpreted in Its Relations to Contemporaneous Religious Currents (1929; reprint, Amsterdam: B. RR. Gruner, 1986), 63


Note that The New International Commentary on the New Testament identifies Odeberg as a proponent of the "water = semen" view, which suggests that the latter citation may not be relevant to the amniotic fluid view. (see p. 191-192) The citation given appears to be the same source:


Hugo Odeberg, The Fourth Gospel, (Uppsala, 1929), 48-71


I also learned from this commentary that Calvin viewed "water and Spirit" as synonymous, and that Luther said that "water" refers to water baptism, which I thought was interesting.

Anyway, I hope that helps point you in the right direction.