The first lifeless phase is the period when one is a baby in a mother's womb before Allah gives him/her the soul. A related ayah that strengthen this understanding is the following:
كَيْفَ تَكْفُرُونَ بِاللَّهِ وَكُنتُمْ أَمْوَاتًا فَأَحْيَاكُمْ ۖ ثُمَّ يُمِيتُكُمْ ثُمَّ يُحْيِيكُمْ ثُمَّ إِلَيْهِ تُرْجَعُونَ
Sahih International Translation
How can you disbelieve in Allah when you were lifeless and He brought you to life; then He will cause you to die, then He will bring you [back] to life, and then to Him you will be returned.
In this ayah, Allah says that we were lifeless and he brought us to life, this happened in the past. And then He will (in the future) cause us to die and then bring us back to life. Obviously, we have so far come to life only once, and before becoming alive, we existed but without being alive only when we were in our mothers' wombs before Allah gave us our souls.
The reason why the translation differs between lifeless and dead is that the Arabic word أموات is used. This word, if translated literally, means dead. However, in Quran, words can have more than their own meanings based on the context and the eloquence.
Tafsir Scholar Al-Shankiti says in his book أضواء البيان في إيضاح القرآن بالقرآن:
والأظهر عندي أن المسوغ الذي سوغ إطلاق اسم الموت على العلقة ، والمضغة مثلا ، في بطون الأمهات ، أن عين ذلك الشيء ، الذي هو نفس العلقة والمضغة ، له أطوار كما قال تعالى : وقد خلقكم أطوارا يخلقكم في بطون أمهاتكم خلقا من بعد خلق [ 39 \ 6 ] ، ولما كان ذلك الشيء ، تكون فيه الحياة في بعض تلك الأطوار ، وفي بعضها لا حياة له ، صح إطلاق الموت والحياة عليه من حيث إنه شيء واحد ، ترتفع عنه الحياة تارة وتكون فيه أخرى
It appears to me that the reason behind referring to an embryo in a womb as dead, is that it (the embryo) has phases of development as Allah says in ayah 6/30: "[..] He creates you in the wombs of your mothers, creation after creation, within three darknesses. [..]". And since the embryo may have life in some phases and not have life in others, then it's correct to point it as either alive or dead (since they are the opposite of each other).
Also there's another opinion by Tafsir Scholar Al-Zamakhshari and many others, but I'm not able to translate it well, you can read it in Arabic here.