IVF With Only 1–2 Eggs: Could 5 Hours With Sperm Be Enough?

When IVF yields only one or two eggs, each fertilization attempt can determine the fate of the entire cycle. Is overnight sperm–egg co-incubation the best approach, or could roughly five hours be enough? A retrospective study of 2,392 conventional IVF cycles found no significant cumulative live birth difference between five-hour and 17-hour insemination, while the shorter protocol had fewer transfer cancellations and cycles with no usable embryos.

The findings appeared in Frontiers in Endocrinology in 2026. For patients with a low ovarian response, they raise a practical question about how to make the most of a very small number of hard-won eggs.

Comparing 2,392 IVF cycles with one or two retrieved eggs

In conventional IVF, a selected concentration of sperm is placed around the retrieved eggs so that sperm can enter the eggs without being injected. Overnight co-incubation has traditionally been common.

The researchers examined 2,392 cycles yielding one or two oocytes. In 2,057 short-term cycles, eggs and sperm were separated after approximately five hours. In 335 overnight cycles, they remained together for approximately 17 hours.

Because the groups differed in patient characteristics, the investigators also used statistical matching to compare patients with more similar baseline conditions.

Cumulative live birth: 20.7% versus 17.9%

After matching, cumulative live birth rates were 20.7% with short-term insemination and 17.9% with overnight insemination. The numerical advantage was not statistically significant.

The study therefore did not find evidence of a lower final birth rate after separation at around five hours. However, a nonsignificant result does not establish equivalence or prove that shortening co-incubation cannot reduce success in other settings.

Fewer cycles ended without an embryo to transfer

The more notable difference appeared in obtaining embryos. In the matched analysis, short-term insemination was associated with lower transfer-cancellation rates and fewer cycles without any usable embryo.

Total fertilization failure, the number of usable embryos and the number of good-quality embryos did not differ significantly between groups.

This distinction matters when few eggs are retrieved. If many are available, another egg may still yield a transferable embryo when one fails to fertilize or stops developing. With only one or two eggs, the outcome of each may determine whether a transfer is possible at all. Obtaining even one usable embryo can therefore be an important outcome alongside live birth.

Longer contact is not necessarily better

It is tempting to assume that leaving eggs and sperm together longer provides more opportunity for fertilization. That is not necessarily so.

Research into shorter co-incubation has considered how prolonged exposure to sperm and surrounding cumulus cells may alter the culture environment and increase reactive oxygen species. Separating eggs once sufficient time for fertilization has passed could potentially improve subsequent culture conditions. These are possible explanations, not proof of the mechanism behind the observed results.

In this study, the shorter protocol was associated with fewer cancellations and cycles without usable embryos, without a statistically significant reduction in cumulative live birth.

Not a rule that every low-yield IVF cycle needs five-hour insemination

The findings do not establish that short-term insemination is best for every patient with one or two eggs. This was a retrospective analysis, not a randomized trial, and the two protocols were used mainly during different periods. Matching reduced measured differences but cannot remove all potential confounding.

The choice of fertilization method still depends on egg maturity, sperm count, motility and morphology, prior fertilization failure, and laboratory conditions and protocols.

The central point is that a low egg yield does not automatically make longer sperm–egg contact safer or more advantageous. The possibility that a roughly five-hour protocol could reduce the chance of ending a hard-started cycle with no embryo deserves further study, while its effect on the chance of birth remains uncertain.

Study source

This article is based on “Short-term vs. overnight insemination: which one is better for patients with only one or two oocytes retrieved in IVF cycles?” by researchers at Xiamen University, China, published in Frontiers in Endocrinology in 2026. DOI: 10.3389/fendo.2026.1647862.