Imago Dei

Basing the right to life on equality of dignity sounds really good at first glance, but it is in reality a terrible argument. Because everyone can intrinsically see that a) equality doesn't really exist, b) that the child is in no quantifiable way equal to the mother. Whether in ability, capability, reason, power, or value to others, or even the mother herself. Ergo equaltarian societies often prioritize the "equality" of the strong over the "equality" of the weak, disabled or unborn.  

Because of this, in the mind of most people the "equality" of the mother will always trump the equality of the child, because the mother has first dibs on life. So basing dignity on equality is in the end fruitless, and falls down in practice. 

Dignity and value must be based on something greater: the imago dei, the image of God. Innocent human beings should not be killed in the womb because of some mythical, easily dismissed concept like equality. But something far more important: God's image is stamped on every human being, and because of this they have intrinsic value, sacred value, to harm this image bearing person is sacrilege. 

You don't have to be equal to be considered valuable, human dignity and value is based on something far more sacred and intrinsic. The sacred stamp of the image of God on all human beings is what makes them invaluable, and makes hurting innocent human beings high blasphemy.

Equality is the force upholding abortion, it's not capable of overcoming it. Five Catholic justices with a radical belief in the imago dei overturned abortion not equalitarian progressives, for a reason.