Embryo transfer refers to a step in the process of assisted reproduction in which embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

s are placed into the uterus

Uterus

The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

 of a female with the intent to establish a pregnancy

Pregnancy

Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

. This technique (which is often used in connection with in vitro fertilization

In vitro fertilisation

In vitro fertilisation is a process by which egg cells are fertilised by sperm outside the body: in vitro. IVF is a major treatment in infertility when other methods of assisted reproductive technology have failed...

 (IVF)), may be used in humans or in animals, in which situations the goals may vary.
Embryos can be either “fresh” from fertilized egg cells of the same menstrual cycle

Menstrual cycle

The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....

, or “frozen”, that is they have been generated in a preceding cycle and undergone embryo cryopreservation

Embryo cryopreservation

Cryopreservation of embryos is the process of preserving an embryo at sub-zero temperatures, generally at an embryogenesis stage corresponding to pre-implantation, that is, from fertilisation to the blastocyst stage.-Indications:...

, and are thawed just prior to the transfer.