On Gen 30:9
וַתֵּ֣רֶא לֵאָ֔ה כִּ֥י עָמְדָ֖ה מִלֶּ֑דֶת וַתִּקַּח֙ אֶת־זִלְפָּ֣ה שִׁפְחָתָ֔הּ וַתִּתֵּ֥ן אֹתָ֛הּ לְיַעֲקֹ֖ב לְאִשָּֽׁה׃
When Leah saw that she had stopped bearing, she took her maid Zilpah and gave her to Jacob as concubine (Sefria translation. I translate wife.).
Sharing your husband with another wife is an act of stress, as we know from Abraham-Sara-Hagar, and from the Rachel, giving Biha to Yakkov only after she is stressed.
His twelve children were born during 7 years, and from simple look at the verses, none of his wives were pregnant simultaneously, so one of his wives was pregnant all the time.
Why Leah is so stressed that she gives her maid to Yaakov as a wife? She has 4 children in a row, other then her only Bilha has kids - only two, and she is not giving birth for only 1.5-2~ years?
What is the rush?