The Meam Loez claims that Yakov Avinu fathered a total of twenty-three children in seven years. There were twin sisters born to most of the tribes. The brothers were permitted to marry sisters of a different mother, according to the Noachide laws in effect at that time.
Leah had six boys and one daughter, that the Torah enumerated. The Midrash wrote about the lesser known twin sisters. Her pregnancies lasted only seven months. And she gave birth to exceptionally strong and healthy babies.
Originally, it was assumed that the four wives of Yakov, would each deliver three of the future tribes of Israel. When Leah had her fourth son, Yehuda, she made a special declaration.
She said, “This time, I will give thanks to Hashem.” The Rabbis commented on this and said that she was the first person to give thanks to G-d.
This statement is surprising as there were many before her, who built altars, and offered sacrifices to G-d. Rav Elazar Abuchatzeira gave an answer to this question.
He said that Leah was the first to give thanks for a natural occurrence. Having a baby, is certainly an every day natural event. Those who preceded Leah, gave thanks for supernatural events. Noach built an alter after the flood. The Patriarchs gave thanks for their miraculous salvation.
Leah taught us the importance of not taking for granted, even natural events. Giving birth to a heathy baby, is not a given. The same is true of all the daily gifts we receive from Above. We are to particularly pay attention to this when we recite the מודים prayer three times every day. Leah taught us not to take anything for granted. The least we can do, is give thanks to G-d for His abundant kindness.