Day 1 Fertilization Results: What is the embryologist looking for?


2 min read

After your eggs are retrieved as part of an IVF procedure, they are cultured in the lab for up to 7  days. The embryology team will check in on their growth periodically using a microscope. They look for certain signs that tell them how each embryo is developing. The first day they examine the embryos is on day 1 of fertilization. Day 1 is the day after your egg retrieval.

Embryo fertilization calendar following egg retrieval

You will likely get a call or update on your patient portal on day 1 informing you how many eggs were fertilized. We’ve got you covered on what the embryologist is looking for and what you can expect to learn about your embryos.

A normally fertilized embryo on day 1 is said to have 2PN (pronuclei) and 2PB (polar bodies). To the average person this means nothing, but these are great signs for an embryo. Here’s what you need to know:

  • Polar Bodies (PB) - Before fertilization, egg cells divide through the process of meiosis and go from 46 chromosomes to 23 chromosomes. Then those 23 chromosomes divide again to create an egg cell with DNA that is ready to pair with the DNA of the sperm. The two portions of unused DNA go into 2 tiny cells outside of the egg cell known as the polar bodies or PB.

  • Pronuclei (PN) - When the sperm penetrates the egg, each cell creates a pronucleus or PN. A pronucleus is made up of a membrane that surrounds DNA. There is one pronuclei for the sperm and one for the egg before they join together to combine the sperm and egg DNA. 

  • The pronuclei typically become visible between 6 and 20 hours after fertilization occurred.

  • The embryologists usually check for the PN and PB between 17 and 20 hours after fertilization.

When the embryologist examines the eggs to see if they have been fertilized, they’re looking for 2 polar bodies and 2 pronuclei. The pronuclei are the most important as they are evidence that the sperm has penetrated the egg and there are two sets of DNA present. 

What does it mean if there aren’t 2 pronuclei seen?

  • 0PN or 1PN - These embryos shouldn’t be counted out right away. Sometimes, they will show normal signs of fertilization later in time than the moment when they were examined by the embryologist.

  • >2PN - These embryos are not compatible with normal embryonic development. More than 2PN means that there is too much DNA in the cell. This is often caused by more than 1 sperm fertilizing the egg.

Note: All embryology labs function differently. Clinics may provide updates on different schedules. Make sure to ask your clinic when and how you will be updated regarding embryo development.