Partial hydatidiform mole is an oddball complication of pregnancy. A complication that most commonly causes a spontaneous abortion or miscarriage of pregnancy. Every women’s eggs have a shell called Zona Pellucida (see Figure below), and multiple small granules called Cortical Granules (see Figure below).
Normally, when one sperm undergoes the acrosome reaction, and can penetrate the zona pellucida egg shell or fertilize the egg, it causes upon penetration the cortical granules to release enzymes into the perivitteline space (see Figure). These enzymes cross link or tie together the glycoproteins that make up the zonal pellucida egg shell, such that it becomes unpenetratable by any further sperm. This way only the the haploid (½ set) set of chromosomes in one sperm can combine with the haploid (½ set) set of chromosome in an egg to make a new diploid (whole set) embryo with no interference.
Approximately 1 time in 141 fertilizations or pregnancies, the cortical granules fail to release the enzymes upon penetration of the zona pellucida. As such the egg can be penetrated by more more than one sperm. The result is that the ½ set from more than one sperm may enter into the fertilization equation, and one ends up with a faulty triploid (1½ sets of chromosomes) embryo. This is a partial mole, it has 2 haploid or half sets of chromosomes from male sperm, and one haploid or half set from the female egg. Somehow, when growth and differentiation start to occur the process goes crazy, and one end up with a mess of shuffled up fetal components and placental components. This is a partial mole. If a partial mole is not miscarried, then it will show up on ultrasound as a partial mole with no clear fetus.
At this time, termination by Dilation and Curettage is recommended. Although in some countries physicians just wait for it to miscarry. Rarely, does a partial hydatididiform mole lead to a persistent or invasive hydatidiform mole.