Sperm and ovum usually unite, if they do, in a woman’s fallopian tube ampulla. This is not a womb.
Sperm and ovum can unite in IVF In vitro fertilisation. This, also, not a womb.
A zygote (from Greek ζυγωτός zygōtos “joined” or “yoked”, from ζυγοῦν zygoun “to join” or “to yoke”) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote’s genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual. In multicellular organisms, the zygote is the earliest developmental stage. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring.
[wiki pedia zygote]
But a zygote develops more than just a fetus…
A cavity forms inside the morula, by the active transport of sodium ions from trophoblast cells and osmosis of water. This results in a hollow ball of cells known as the blastocyst. The blastocyst’s outer cells will become the first embryonic epithelium (the trophectoderm). Some cells, however, will remain trapped in the interior and will become the inner cell mass (ICM), and are pluripotent. In mammals (except monotremes), the ICM will ultimately form the “embryo proper”, while the trophectoderm will form the placenta and extra-embryonic tissues.
The hypoblast contributes to extraembryonic membranes and the epiblast will give rise to the ultimate embryo proper as well as some extraembryonic tissues.
[wikipedia inner cell mass]
The implantation window is characterized by changes to the endometrium cells, which aid in the absorption of the uterine fluid. These changes are collectively known as the plasma membrane transformation and bring the blastocyst nearer to the endometrium and immobilize it. During this stage the blastocyst can still be eliminated by being flushed out of the uterus.
[wikipedia implantation (human embryo]
Conceptus (Latin: conceptio, meaning derivatives of zygote) denotes the embryo and its adnexa (appendages or adjunct parts) or associated membranes (i.e. the products of conception). The conceptus includes all structures that develop from the zygote, both embryonic and extraembryonic. It includes the embryo as well as the embryonic part of the placenta and its associated membranes - amnion, chorion (gestational sac), and yolk sac.
In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus.
But these are technical terms; they will not give you wisdom about rebirth.
Some embryos do not survive to the next stage of development. When this happens naturally, it is called spontaneous abortion or miscarriage. There are many reasons why this may occur. The most common natural cause of miscarriage is chromosomal abnormality in animals or genetic load in plants.
In species which produce multiple embryos at the same time, miscarriage or abortion of some embryos can provide the remaining embryos with a greater share of maternal resources. This can also disturb the pregnancy, causing harm to the second embryo. Genetic strains which miscarry their embryos are the source of commercial seedless fruits.
Abortion is the process of artificially (non-naturally) removing the embryo through deliberate pharmaceutical or surgical methods.
Abortion may or may not be against the first precept; an embryo which is dead could be removed, for health of the mother, or (possibly) a twin still in utero.
Miscarriage may or may not mean the death of a being; we do not know when rebirth occurs, we do not know if miscarried cells ever carried cita simply based on technical terms of biology or medicine.
A living limb may be amputated without breaking the first precept; the cells in that limb will DIE but this is not “taking a life”. A miscarriage may occur without breaking the first precept; menstruation can occur without breaking the first precept. A still birth may occur without breaking the first precept. Ectopic pregnancy may occur; maternal death in childbirth may occur.
The first precept imo is best kept, maybe only kept, BY ONESELF. All the precepts are DIY. If you are a human male, as far as I know, you cannot get pregnant, but you can impregnate. So - if you are male, consider a vasectomy; it is usually easily reversible, and is simple operation with few if any side effects. You might also want to educate yourself and other males about male birth control (in addition to brahmacariya, which may not be for everyone, despite its excellent side benefits). https://www.malecontraceptive.org/male-contraception-faq/
Trying to conform the first precept of the Buddha to political ideology or abrahamic religuous concepts is imo folly. It seems to encourage beliefs in souls and selfs.