From the Writings and Utterances of `Abdu’l-Baha:
Question. – It is recorded that miracles were performed by Christ. Are the reports of these miracles really to be accepted literally, or have they another meaning? It has been proved by exact science that the essence of things does not change, and that all beings are under one universal law and organization from which they cannot deviate; and, therefore, that which is contrary to universal law is impossible.
Answer. – The Holy Manifestations are the sources of miracles and the originators of wonderful signs. For Them, any difficult and impracticable thing is possible and easy. For through a supernatural power wonders appear from Them; and by this power, which is beyond nature, They influence the world of nature. From all the Manifestations marvelous things have appeared.
But in the Holy Books an especial terminology is employed, and for the Manifestations these miracles and wonderful signs have no importance. They do not even wish to mention them. For if we consider miracles a great proof, they are still only proofs and arguments for those who are present when they are performed, and not for those who are absent.
(`Abdu’l-Baha: Some Answered Questions, p. 100)

The meaning is not that the Manifestations are unable to perform miracles, for They have all power. But for Them inner sight, spiritual healing and eternal life are the valuable and important things. Consequently, whenever it is recorded in the Holy Books that such a one was blind and recovered his sight, the meaning is that he was inwardly blind, and that he obtained spiritual vision, or that he was ignorant and became wise, or that he was negligent and became heedful, or that he was worldly and became heavenly.
(`Abdu’l-Baha: Some Answered Questions, p. 102)

I do not wish to mention the miracles of Baha’u’llah, for it may perhaps be said that these are traditions, liable both to truth and to error, like the accounts of the miracles of Christ in the Gospel, which come to us from the apostles, and not from anyone else, and are denied by the Jews. Though if I wish to mention the supernatural acts of Baha’u’llah, they are numerous; they are acknowledged in the Orient, and even by some non-Baha’is. But these narratives are not decisive proofs and evidences to all; the hearer might perhaps say that this account may not be in accordance with what occurred, for it is known that other sects recount miracles performed by their founders. For instance, the followers of Brahmanism relate miracles. From what evidence may we know that those are false and that these are true? If these are fables, the others also are fables; if these are generally accepted, so also the others are generally accepted. Consequently, these accounts are not satisfactory proofs. Yes, miracles are proofs for the eyewitness only, and even he may regard them not as a miracle but as an enchantment. Extraordinary feats have also been related of some conjurors.
Briefly, my meaning is that many wonderful things were done by Baha’u’llah, but we do not recount them, as they do not constitute proofs and evidences for all the peoples of the earth, and they are not decisive proofs even for those who see them: they may think that they are merely enchantments.
Also, most of the miracles of the Prophets which are mentioned have an inner significance. For instance, in the Gospel it is written that at the martyrdom of Christ darkness prevailed, and the earth quaked, and the veil of the Temple was rent in twain from the top to the bottom, and the dead came forth from their graves. If these events had happened, they would indeed have been awesome, and would certainly have been recorded in the history of the times. They would have become the cause of much troublings of heart. Either the soldiers would have taken down Christ from the cross, or they would have fled. These events are not related in any history; therefore, it is evident they ought not to be taken literally, but as having an inner significance.(1)
Our purpose is not to deny such miracles; our only meaning is that they do not constitute decisive proofs, and that they have an inner significance.
(`Abdu’l-Baha: Some Answered Questions, pp. 37-38)

If we wish to discover whether any one of these great Souls or Messengers was in reality a Prophet of God, we must investigate the facts surrounding His life and history, and the first point of our investigation will be the education He bestowed upon mankind. If He has been an Educator, if He has really trained a nation or people, causing it to rise from the lowest depths of ignorance to the highest station of knowledge, then we are sure that He was a Prophet. This is a plain and clear method of procedure, proof that is irrefutable. We do not need to seek after other proofs. We do not need to mention miracles, saying that out of rock water gushed forth, for such miracles and statements may be denied and refused by those who hear them. The deeds of Moses are conclusive evidences of His Prophethood.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 364)

From the Writings of Shoghi Effendi:
The operation of miracles is not necessarily irrational or illogical. It does by no means constitute a limitation of the Omnipotence of God. The belief in the possibilities of miracles, on the contrary, implies that God’s power is beyond any limitation whatsoever. For it is only logical to believe that the Creator, Who is the sole Author of all the laws operating in the universe, is above them and can, therefore, if He deems it necessary, alter them at His Own Will.
(Shoghi Effendi: Lights of Guidance, p. 491)

From Letters Written on Behalf of Shoghi Effendi:
First regarding the birth of Jesus Christ. In light of what Baha’u’llah and Abdu’l-Baha have stated concerning this subject it is evident that Jesus came into this world through the direct intervention of the Holy Spirit, and that consequently His birth was quite miraculous. This is an established fact, and the friends need not feel at all surprised, as the belief in the possibility of miracles has never been rejected in the Teachings. Their importance, however, has been minimized.
(Shoghi Effendi: Lights of Guidance, p. 490)

Again with regard to your question relative to the birth of Jesus: He wishes me to inform you that there is nothing further he can add to the explanation he gave you in his previous communication regarding this point. One thing, however, he wishes again to bring to your attention, namely that miracles are always possible, even though they do not constitute a regular channel whereby God reveals His power to mankind. To reject miracles on the ground that they imply a breach of the laws of nature is a very shallow, well-nigh a stupid argument, inasmuch as God Who is the Author of the universe can, in His Wisdom and Omnipotence, bring any change, no matter how temporary, in the operation of the laws which He Himself has created.
(Shoghi Effendi: Lights of Guidance, p. 490)

1. Cf. “Miracles,” p. 100.