Glossary of Reproductive Treatments Terminology

The following glossary was developed by the World Health Organization (WHO) as an internationally accepted set of definitions to monitor the availability, efficacy, and safety of interventions in reproductive medicine.

Assisted Hatching: An in vitro procedure in which the zona pellucida (eggshell) of an embryo is perforated by chemical, mechanical, or laser methods to assist hatching of the blastocyst.

Assisted reproductive technology (ART): All treatments or procedures that include the in vitro handling of both human oocytes (eggs) and sperm or of embryos for the purpose of establishing a pregnancy. This includes, but is not limited to, in vitro fertilization and embryo transfer, gamete intrafallopian transfer, zygote intrafallopian transfer, tubal embryo transfer, gamete and embryo cryopreservation, oocyte and embryo donation, and gestational surrogacy.

Biochemical pregnancy: A pregnancy diagnosed only by the detection of hCG (pregnancy hormone) in serum or urine and that does not develop into a clinical pregnancy.

Blastocyst: An embryo (80 or more cells), 5 or 7 days after fertilization, with an inner cell mass, outer layer of trophectoderm (will form the embryonic side of the placenta ), and a fluid-filled blastocele cavity at the center.

Clinical pregnancy: A pregnancy diagnosed by visualization of one or more gestational sacs on an ultrasound or definitive clinical signs of pregnancy. It includes ectopic pregnancy. Note: Multiple gestational sacs are counted as one clinical pregnancy.

Clinical pregnancy rate: The number of clinical pregnancies expressed per 100 initiated cycles, aspiration cycles, or embryo transfer cycles. Note: When clinical pregnancy rates are given, the denominator (initiated, aspirated, or embryo transfer cycles) must be specified.

Clinical pregnancy with fetal heart beat: Pregnancy diagnosed by ultrasonographic or clinical documentation of at least one fetus with heart beat. It includes ectopic pregnancy.

Controlled ovarian stimulation for ART: Pharmacologic treatment in which ovaries are stimulated to induce the development of multiple ovarian follicles to obtain multiple oocytes (eggs).

Cryopreservation: The freezing or vitrification and storage of gametes (sperm and eggs), zygotes (fertilized egg), embryos, or ovarian/testicular tissue.

Delivery rate: The number of deliveries expressed per 100 initiated cycles, aspiration cycles, or embryo transfer cycles. When delivery rates are given, the denominator (initiated, aspirated, or embryo transfer cycles) must be specified. It includes deliveries that resulted in the birth of one or more live babies and/or stillborn babies. Note: The delivery of a singleton, twin, or other multiple pregnancy is registered as one delivery.

Ectopic pregnancy: A pregnancy in which implantation takes place outside the uterine cavity.

Embryo: The product of the division of the zygote (fertilized egg) to the end of the embryonic stage, 8 weeks after fertilization.

Embryo donation: The transfer of an embryo resulting from gametes (sperm and eggs) that did not originate from the recipient and her partner.

Embryo recipient cycle: An ART cycle in which a woman receives embryo(s) from donor(s).

Embryo/fetus reduction: A procedure to reduce the number of viable embryos or fetuses in a multiple pregnancy.

Embryo transfer (ET): The procedure in which one or more embryos are placed in the uterus or fallopian tube.

Fertilization: The penetration of the ovum by the sperm and combination of their genetic material resulting in the formation of a zygote (fertilized egg).

Fetus: The product of fertilization from completion of embryonic development, at 8 completed weeks after fertilization, until birth.

Frozen-thawed embryo transfer cycle (FET): An ART procedure in which uterine stimulation/cycle monitoring is carried out with the intention of transferring frozen-thawed embryo(s).

Gestational carrier (surrogate): A woman who carries a pregnancy with an agreement that she will give the offspring to the intended parent(s). Gametes (eggs and sperm) can originate from the intended parent(s) and/or a third party (or parties).

Gestational sac: A fluid-filled structure associated with early pregnancy, which may be located inside or outside the uterus (in case of an ectopic pregnancy).

Hatching: The process by which an embryo at the blastocyst stage separates from the zona pellucida (eggshell).

High-order multiple: A pregnancy or delivery with three or more fetuses or neonates.

Implantation: The attachment and subsequent penetration by the zona-free (after hatching out of its eggshell) blastocyst (usually in the endometrium-uterine lining) that starts 5 to 7 days after fertilization.

Implantation rate: The number of gestational sacs observed divided by the number of embryos transferred.

In vitro fertilization (IVF): An ART procedure that involves extracorporeal fertilization

Infertility (clinical definition): A disease of the reproductive system defined by the failure to achieve a clinical pregnancy after 12 months or more of regular unprotected sexual intercourse.

Initiated cycle: An ART cycle in which the woman receives specific medication for ovarian stimulation, or monitoring in the case of natural cycles, with the intention to treat, irrespective of whether or not follicular aspiration is attempted.

Intracytoplasmic sperm injection (ICSI): A procedure in which a single sperm is injected into the oocyte cytoplasm (inside the egg).

Live birth delivery rate: The number of deliveries that resulted in at least one live born baby, expressed per 100 initiated cycles, aspiration cycles, or embryo transfer cycles. When delivery rates are given, the denominator (initiated, aspirated, or embryo transfer cycles) must be specified.

MESA: Microsurgical epididymal (convoluted duct behind the testis) sperm aspiration.

Micromanipulation: A technology that allows micro-operative procedures to be performed on the sperm, oocyte (egg), zygote (fertilized egg), or embryo.

Mild ovarian stimulation for IVF: A procedure in which the ovaries are stimulated with gonadotropins (injectable medications) and/or other compounds, with the intent to limit the number of oocytes obtained for IVF to fewer than seven.

Natural cycle IVF: An IVF procedure in which one or more oocytes (eggs) are collected from the ovaries during a spontaneous menstrual cycle without any drug use.

Oocyte donation cycle: Donor Egg IVF, a cycle in which oocytes (eggs) are collected from a donor for clinical application or research.

Oocyte recipient cycle: An ART cycle in which a woman receives oocytes (eggs) from a donor.

Ovarian hyperstimulation syndrome (OHSS): An exaggerated systemic response to ovarian stimulation characterized by a wide spectrum of clinical and laboratory manifestations. It is classified as mild, moderate, or severe according to the degree of abdominal distention, ovarian enlargement, and respiratory, hemodynamic, and metabolic complications.

Ovulation induction (OI): Pharmacologic treatment of women with anovulation (lack of ovulation) or oligo-ovulation (infrequent ovulations) with the intention of inducing normal ovulatory cycles.

PESA: Percutaneous epididymal (convoluted duct behind the testis) sperm aspiration.

Preimplantation genetic testing (PGT): Analysis of embryos for the detection of genetic, structural, and/or chromosomal alterations.

Recurrent spontaneous abortion/miscarriage: The spontaneous loss of two or more clinical pregnancies.

Severe ovarian hyperstimulation syndrome: Severe OHSS is defined to occur when hospitalization is indicated. (See definition of "ovarian hyperstimulation syndrome.")

TESA: Testicular sperm aspiration.

Vanishing sac(s) or embryo(s): Spontaneous disappearance of one or more gestational sacs or embryos in an ongoing pregnancy, documented by ultrasound.

Vitrification: An ultra-rapid cryopreservation method that prevents ice formation

Zygote: A diploid cell resulting from the fertilization of an oocyte (egg) by a sperm, which subsequently divides to form an embryo.