Researchers observed asymmetric follicle growth in women with reduced ovarian reserve. Among 18 women with a response on only one side, 15 responded on the left.
Egg maturation, fertilization, embryo development and pregnancy measures were similar between sides. More growing follicles do not necessarily mean better eggs.
There are normally two ovaries, one on each side, supporting follicle growth and egg release. Do they always function similarly? In particular, do both respond alike as women age or ovarian reserve declines?
Researchers including a team at Juntendo University in Japan analyzed ovarian stimulation in women with impaired ovarian function and observed differences in follicle growth between the two ovaries. The left side predominated in a small subgroup with follicle growth on only one side.
The study appears in the November 2026 issue of Reproductive BioMedicine Online.
Two ovaries do not always mean equal follicle growth
In IVF, ovarian stimulation medication is used to encourage the growth of multiple follicles. Ultrasound monitoring tracks their numbers and sizes in both ovaries.
The same medication does not always produce an identical response on each side. Several follicles may grow in the right ovary and only one or two in the left, or the opposite pattern may occur.
The researchers examined 2,181 ovarian stimulation episodes in women with primary ovarian insufficiency or diminished ovarian reserve.
Asymmetry in follicle growth was not uncommon. Of 18 women in whom follicle growth occurred on only one side, 15 had growth in the left ovary.
This finding cannot readily be generalized to all women. The unilateral-response subgroup was small, and the study concerned a specific population with impaired ovarian function.
Does a stronger left-sided response mean better egg quality?
A key finding was that different follicle growth did not necessarily translate into different egg-related outcomes.
Comparisons of oocyte maturation, fertilization, good-quality embryo formation and clinical pregnancy rates showed no clear differences between sides.
More follicles growing in one ovary therefore does not establish that its eggs are of higher quality.
IVF assessment involves more than counting growing follicles. It also considers how many mature eggs are retrieved, whether normal embryo development follows fertilization and whether treatment ultimately leads to pregnancy and live birth.
What could this mean for reduced ovarian reserve?
Women with low ovarian reserve may yield fewer eggs after stimulation. Careful observation of both ovaries is therefore important.
This study shows that the ovaries need not respond identically. A limited response on one side may still leave the possibility of obtaining eggs from the other.
The findings do not establish that the left ovary is inherently superior or that it should be preferentially stimulated.
Nor does this study alone provide a basis for decisions about ovarian tissue preservation or the side of ovarian surgery.
Declining ovarian function may not progress in precisely the same way on both sides. Alongside overall ovarian reserve, the actual response of each ovary may deserve attention.
Source: A study of asymmetry in ovarian follicle growth by researchers including Juntendo University, Japan. Reproductive BioMedicine Online, published online July 24, 2026. DOI: 10.1016/j.rbmo.2026.105884. It appears in the November 2026 issue but was not first published in November. Findings do not guarantee an individual's treatment outcome; stimulation and retrieval plans require specialist consultation.
Related reading: How follicle growth helps determine ovulation timing.
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