Hagar – How Many Pregnancies?

Exegetical Approaches

Commentators present two basic approaches to understanding the words "הִנָּךְ הָרָה" and Hagar's state:

Hagar had Miscarried

Sarai had cast an evil eye upon Hagar causing her to miscarry. The angel, thus, informs her that if she returns to Avram she will conceive once again and bear a child.

Omnisignificance – This approach is motivated by the seeming superfluity in the angel's comments. The Midrash views all words of the Biblical text as pregnant with meaning, leaving no room for redundancies.
Tense of "הִנָּךְ הָרָה" – This position assumes that the phrase is referring to a future event, and the angel is telling Hagar that she will become pregnant after returning home. As הנה clauses always take present tense verbs even when they refer to the immediate future, the fact that "הָרָה" is the present form of the participle does not preclude this read.1
Shofetim 13 - Rashi compares our verse to the similar ones in Shofetim 13,2 where the angel uses the same language to refer to a future conception.3
"וַתְּעַנֶּהָ שָׂרי" – There is no attempt here to defend or mitigate Sarai's harsh treatment of Hagar. In fact, the suggestion of Sarai causing the miscarriage exacerbates it.
The evil eye – The Midrash derives the concept of the use of the evil eye from an extra yud in the word "בֵינֶיך" (normally spelled defectively.) It is possible, though, that the idea is also a play on Avraham's words, "עֲשִׂי לָהּ הַטּוֹב בְּעֵינָיִךְ"‎.4

Entering into Conversation

Hagar is currently pregnant, and the angel is simply stating an already known fact in order to lead up to the next part of the conversation.

Redundancy – Radak suggests that the angel's repetition is a natural part of everyday speech. At times, one states what is already apparent simply as a way of making conversation, or of steering it in a particular direction.5 This reading fits with Radak's general approach which allows for repetition in the Biblical text, explaining such cases as either being literary techniques or reflecting the mannerisms of people.
Tense of "הִנָּךְ הָרָה" – According to Ibn Ezra, Radak, and Ralbag, the phrase is making a statement about Hagar's present condition; the angel reminds her that she is presently expecting a child.6
Sarai's behavior – This position does not make any evaluation of Sarai's actions.
Yeshayahu 7:14 and polemics The decision to read the clause in the present tense might be related to the similar ambiguity regarding the almost identical clause in Yeshayahu 7:14: "לָכֵן יִתֵּן אֲדֹנָי הוּא לָכֶם אוֹת הִנֵּה הָעַלְמָה הָרָה וְיֹלֶדֶת בֵּן". As this verse is understood by Christians to be a prophecy later fulfilled by the virgin birth of Jesus,7 Radak and others prefer to distance themselves from that possibility by positing that the prophet is not talking about a future conception, but a present pregnancy (of a married woman).8