Gad, Asher, and Dinah: absolutely, totally, and in all other ways inconceivable?

Question

In parashas "Vayetze" (Gen. 29–30) we read about the birth of twelve children of Yaakov's:

1. וַתַּהַר לֵאָה וַתֵּלֶד בֵּן וַתִּקְרָא שְׁמוֹ רְאוּבֵן….‏
2. וַתַּהַר עוֹד וַתֵּלֶד… וַתִּקְרָא שְׁמוֹ שִׁמְעוֹן.‏
3. וַתַּהַר עוֹד וַתֵּלֶד… קָרָא שְׁמוֹ לֵוִי.‏
4. וַתַּהַר עוֹד וַתֵּלֶד… קָרְאָה שְׁמוֹ יְהוּדָה….‏
5. וַתַּהַר בִּלְהָה וַתֵּלֶד… קָרְאָה שְׁמוֹ דָּן.‏
6. וַתַּהַר עוֹד וַתֵּלֶד… וַתִּקְרָא שְׁמוֹ נַפְתָּלִי.‏
7. וַתֵּלֶד… וַתִּקְרָא אֶת שְׁמוֹ גָּד.‏
8. וַתֵּלֶד… וַתִּקְרָא אֶת שְׁמוֹ אָשֵׁר.‏
9. וַתַּהַר וַתֵּלֶד… וַתִּקְרָא שְׁמוֹ יִשָּׂשכָר.‏
10. וַתַּהַר עוֹד לֵאָה וַתֵּלֶד… וַתִּקְרָא אֶת שְׁמוֹ זְבֻלוּן.‏
11. וְאַחַר יָלְדָה בַּת וַתִּקְרָא אֶת שְׁמָהּ דִּינָה.‏
12. וַתַּהַר וַתֵּלֶד… וַתִּקְרָא אֶת שְׁמוֹ יוֹסֵף….‏

About all of them, it mentions that the mother got pregnant ("וַתַּהַר") — except Gad and Asher (Zilpa's sons) and Dina. Why?

Answer 1

Rashi (Bereshis 30, pasuk 10) answers on Zilpah that she was very young and the pregnancy was not visible, therefore the verse that she got pregnant was not cited.

Regarding Dinah, it could be like Rashi (Bereshis 30, pasuk 21) that Dina should have been a son and Lea prayed that Rachel should be like the other maids so that she was converted to a girl.
In order to have a sign that this was not a natural pregnancy the Torah omitted the word "vatahar".

Update:

A nice pshat regarding dina is brought by ibn ezra(Bereshis 30, pasuk 21) that dina was actually a twin of zevulun and therefore the word vatahar does not occur. It was not a different pregnancy