Discrepancy between description of how Bilhah and Zilpah had children

Question

I noticed three inconsistencies between how the Torah describes the conception and delivery of the children of Bilhah and Zilpha, only 5 verses apart.

The conception/birth of Bilhah's son Dan is described (Genesis 30:5):

וַתַּ֣הַר בִּלְהָ֔ה וַתֵּ֥לֶד לְיַעֲקֹ֖ב בֵּֽן׃

Bilhah conceived and bore Jacob a son.

The conception/birth of Bilhah's son Naftali is described (Genesis 30:7):

וַתַּ֣הַר ע֔וֹד וַתֵּ֕לֶד בִּלְהָ֖ה שִׁפְחַ֣ת רָחֵ֑ל בֵּ֥ן שֵׁנִ֖י לְיַעֲקֹֽב׃

Rachel’s maid Bilhah conceived again and bore Jacob a second son.

The birth of Zilpah's son Gad is described (Genesis 30:10):

וַתֵּ֗לֶד זִלְפָּ֛ה שִׁפְחַ֥ת לֵאָ֖ה לְיַעֲקֹ֥ב בֵּֽן׃

And when Leah’s maid Zilpah bore Jacob a son,

The birth of Zilpah's son Asher is described (Genesis 30:12):

וַתֵּ֗לֶד זִלְפָּה֙ שִׁפְחַ֣ת לֵאָ֔ה בֵּ֥ן שֵׁנִ֖י לְיַעֲקֹֽב׃

When Leah’s maid Zilpah bore Jacob a second son,

As we can see, with Bilhah the Torah

1. Says ותהר, she conceived, and ותלד, birthing
2. Only mentions that she's שפחת רחל the second time, not the first
3. Adds the word עוד the second time

Whereas, with Zilpah the Torah

1. Only says ותלד, birthing, and no mention of conceiving
2. Calls her שפחת לאה both times
3. Doesn't say the word עוד

Does anyone explain this phenomenon?

Answer 1

The Midrash in Bereishis Rabbah 71:9 notes:

וַתֵּלֶד זִלְפָּה שִׁפְחַת לֵאָה (בראשית ל, י), בְּכֻלָּם כְּתִיב וַתַּהַר, וְכָאן וַתֵּלֶד, אֶלָּא בַּחוּרָה הָיְתָה וְלֹא הָיְתָה נִכֶּרֶת בְּעִבּוּרָהּ

"And when Zilpah, the maidservant of Leah bore" (Bereishis 30:10) - with all of them (i.e. the other three wives) it writes "And she conceived", and here (by Zilpah) it only writes "And she bore", because she was so young that her pregnancy was not noticeable.

Therefore Zilpah only has the expression "וַתֵּלֶד" used due to her relative youth and the resulting physical indistinctness of her pregnancy.

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EDIT

The Meshech Chochmah helps give another option as to why Zilpah's pregnancy was unnoticeable and that it why it warranted just the expression that 'she gave birth':

ותלד זלפה. לא נאמר ותהר בבני זלפה יתכן כי נולדו מהריון אחד וגד נגמר צורתו לשבעה חדשים. (כמספר גד) ואשר נגמרה צורתו לתשעה כיהודה וחזקי' בני ר"ח בנדה כ"ז...

And Zilpah gave birth - It does not say about the children of Zilpah that 'she conceived'. It is possible that they were born from one pregnancy and Gad's form was completed after 7 months (as hinted to by numerical value of Gad [gimmel=3 + dalet=4} - 7), and Asher's form was completed after nine months. It is like Yehuda and Chizkiya, the sons of Rabbi Chiya (Niddah 27a - where they were twins and one child remained in the womb after the other was born for three months)....

So from the Meshech Chochmah (although as he concludes, it is a little difficult to understand - 'ודוחק קצת') we can suggest that it wasn't a noticeable pregnancy as it was all part of one pregnancy and her second child appeared not long after first.