A Frozen Embryo’s Journey from Freezing to Warming

When an embryo is frozen at one clinic and warmed for transfer at another, two laboratories and a carrier each handle part of the process. Their records determine what can be established later. For example, a receiving clinic may verify the carrier count and labels, then discover at warming that the embryo cannot be located on the carrier tip. That observation is the starting point for an investigation.

Cryocarriers and dry shippers

A day-five or day-six blastocyst is roughly 100–200 micrometres across. Vitrification uses cryoprotectants and rapid cooling to minimise ice crystal formation. The embryo is placed on a dedicated device and subsequently kept in cryogenic storage. With an open carrier, the tiny droplet containing the embryo contacts liquid nitrogen directly and a protective cover is fitted afterwards. Closed carriers isolate the specimen from the liquid nitrogen. Sealing and warming procedures depend on the device.[1,8]

Embryos are shipped in dry liquid nitrogen shippers. The absorbent lining holds the liquid nitrogen and cold nitrogen vapour maintains the temperature. One model cited in the original article is rated to remain below −150°C for about ten days under specified conditions. For a particular shipment, check the model, the preparation of the shipper and the temperature log.[5,8]

A 2023 retrospective study compared 621 blastocysts warmed in the same Italian laboratory. The clinic had frozen 450 on site and received 171 from Spanish clinics in the same network.[2]

None of these comparisons reached statistical significance. The P value for clinical pregnancy was 0.06. The study reported no live-birth outcome and evaluated shipping within one clinic network. In a separate mouse study, 96 hours in the vapour phase of a dry shipper did not worsen the measured viability outcomes. Recovery after warming differed by device in that experiment. Those animal recovery rates cannot be applied to human embryos.[2,3]

Five stages from freezing to warming

The UK regulator HFEA documented incidents in which unwitnessed movements or delayed updates to storage records led clinics to report specimens as missing. When identity, counts or storage location do not match, ask the laboratories to reconcile the dispatched, received, warmed and remaining inventory.[4]

A temperature log documents the thermal conditions. To investigate a loose or detached carrier or cover, examine the physical device and records of handling, inspections and impacts. Warming involves opening the device and handling the specimen promptly under the laboratory’s protocol for that carrier.[1,8]

What receipt checks can establish

At receipt, the clinic can inspect the packaging, reconcile carrier counts and labels, and review transport temperatures. The embryo inside a carrier is generally visible only at warming. A normal receipt check and a later finding at warming can both be accurate. The material evidence and records from both sides of the handoff help establish when the problem arose.

The location narrows the inquiry. Damage to the device, photographs, storage and transport records, and the findings at warming together determine what the evidence supports.

When the embryo cannot be located at warming

The laboratory can investigate the identity, location and physical evidence in parallel. While the cause is being investigated, ask the laboratory to keep the other embryos in storage. An additional embryo should not be warmed solely to test whether the problem recurs. The investigation and evidence handling belong with qualified laboratory staff.[1]

1. Retain the carrier, cover, relevant warming solution and other material that could contain the specimen. Document their condition.
2. Have the embryology team examine the material, storage location and identification records for other carriers. If a specimen is recovered, the team assesses its condition.
3. Reconcile each carrier and the embryo counts across release, shipment, receipt, warming and the remaining inventory.
4. Request the freezing, storage, shipping, receipt and warming records from each party, including the original temperature trace and signed handoffs.
5. Request written incident reports. An independent embryology review may help before discussing remedies and insurance under the relevant agreements.

One laboratory or a transfer between clinics

Keeping freezing, storage and warming in one laboratory reduces moves and handoffs and keeps the records together. With two laboratories, check whether the receiving team can warm the carrier used by the sending laboratory, who retains each record, who coordinates an investigation, and how arrival fits with endometrial preparation.

Storage monitoring matters in either arrangement. The 2018 Cleveland and San Francisco incidents cited in the original article involved storage tank failures.[6,7] The American Society for Reproductive Medicine (ASRM) recommends electronic monitoring, preferably with liquid nitrogen level probes, tests at least quarterly and a written emergency plan.[1]

What to agree before dispatch

• Carrier and warming protocol. The sending clinic lists the model, lot and carrier and embryo counts. The receiving laboratory confirms it can use the corresponding warming protocol.
• Freezing and release records. Verify identification, fixation checks and witnessing. If photographs exist, agree how to retain and retrieve them.
• Transport records. Specify the shipper ID, temperature log, signed handoffs and reporting for delays, inspections or incidents.
• Receipt and warming. Identify the checks, the storage location, the operator and witness, and who keeps the reports.
• Insurance and incident handling. Document coverage, each party’s responsibilities, deadlines for providing records and arrangements for independent review.

Build time between shipping and transfer for receipt and verification. Once the embryos are in storage and the identity, counts and temperature record have been checked, the clinical team can coordinate the start of endometrial preparation. Follow the written arrangements agreed with the laboratories and treating team.

Medical information and illustrations

This article explains embryo shipping and its records for educational purposes. The laboratories and treating clinician assess warming, incident investigation and transfer arrangements. The artwork is conceptual.