An LH surge is a rapid increase in luteinizing hormone, which triggers the body’s release of a mature egg. This usually happens in the middle of a woman’s menstrual cycle. A woman is most likely to become pregnant when having intercourse 24 to 48 hours after the LH surge.
Luteinizing hormone signals the follicle to release the mature egg into the Fallopian tube for fertilisation.
Understanding when and how the body ovulates is the key component in taking charge of fertility. Women who are trying to become pregnant may gauge their LH surge through the use of ovulation predictor kits.
The complete menstrual cycle varies from 25 to 36 days and is made up of three distinct phases, all controlled by different hormones.
The first phase is the follicular phase, which begins on day 1 of menstrual bleeding. This is when the pituitary gland starts upping the production of follicle-stimulating hormone, which may produce up to 30 follicles at a time. Each follicle produces an egg, but only one egg will fully mature and produce oestrogen. The follicular phase averages 14 days but may vary from cycle to cycle.
Next comes the ovulatory phase, which begins on the day of the LH surge. The increase in luteinizing hormone sends a message to the follicle to release the mature egg into the Fallopian tube.
And last starts the luteal phase. After release of the egg, the follicle closes and creates a 2- to 5-cm structure called the corpus luteum, which is essential in maintaining pregnancy. The corpus luteum secretes the hormones progesterone and oestrogen, which prepare the uterus to nourish a foetus. The high levels of oestrogen and progesterone also inhibit the production of luteinizing hormone, so an LH surge during pregnancy is not possible.
Not all menstrual cycles are the same. It is possible to have an LH surge and not release an egg, as well as absence of the LH surge altogether. It may also be possible to release more than one mature egg during the ovulatory phase, but this is an extreme exception to the rule.
Some women may feel symptoms of a second LH surge and test after already becoming pregnant. It is possible to get a positive result as a result of pregnancy and not an LH surge. The test will indicate a positive result if the hormone human chorionic gonadotrophin is present in the urine. This is important to note because early detection of pregnancy leads to prompt prenatal care, a proven benefit for a healthy pregnancy.