I did hesitate. I understand that my perspective is neither a female nor a feminist one, but I am surprised you seem to see it as "especially regressive". My previous post was mainly to make the point that is, I think, inherent in most western abortion law, that a zygote/foetus's value is usually in practice not seen as zero (no value) or one (the same value as a live baby). It is usually seen as something in between, with very little value in the first few days after conception (however that is defined) and increased value the more developed it becomes.
That concept does not depend on a perfect understanding or definition of conception. As previously mentioned, quickening and viability may be relevant. They certainly inform the mother's emotional attachment to the developing child. I have no doubt that late miscarriages and stillbirths are typically much more distressing than early miscarriages, which I am told may not always be noticed if very early. That distress partly reflects the mother's relationship with the child she is carrying.
My thoughts on this are as a father. My DW had a far closer pre-birth relationship with our children than I did. It was at birth (or maybe just before) that they became real to me; for my DW they were very real to her much earlier. Not at conception, which as you rightly say was not possible to pinpoint, though we had a pretty good idea.
When abortion law is formulated, it is practicality that matters more than precise definition of conception. I still maintain that this has parallels in death, for example with resuscitation of people whose heart and breathing have stopped, and with life support decisions. We have developed working definitions to help with ethical dilemmas. It is not generally seen as murder to fail to attempt to resuscitate someone who is not breathing and has no detectable pulse, but hospitals have protocols to manage their responsibilities and many people observe with horror "do not resuscitate" notices imposed on a patient.
The idea that for the purposes of abortion law we need a precise date of conception renders abortion law impossible. Only access to abortion at any stage of pregnancy for any reason would be possible, as no limit could be imposed by law.