Your uterus holds the baby until it’s ready to be born. During the first half of your menstrual cycle, estrogen tells your uterus to prepare a “nursery” for a possible egg, by thickening and filling up with extra blood vessels. By mid-cycle, your uterus is ready. That gives the fertilized egg two weeks to arrive from the fallopian tubes and implant. If no fertilized egg arrives, your hormone levels will drop and menstruation will flush out that unused extra uterine lining (also known as your period).

How the uterus actually works

Your uterus sits quite low in your abdomen. It’s held in place by muscles and ligaments. It connects to two fallopian tubes (near its top) and to your cervix (at the bottom), which in turn connects to your vagina.

Once the egg has been released from your ovary and fertilized in your fallopian tube, it travels into the uterus. Throughout the first two weeks of your cycle, the release of estrogen causes the uterus to prepare a “nursery” for a potential embryo: thickening and enriching its lining with extra blood vessels and tissue. Then the hormone progesterone maintains that lining, keeping it nice and thick for two weeks while your uterus waits for the arrival of the fertilized egg.

This allows the egg to burrow into the wall, and connect to your blood system!

Usually the fertilized egg reaches your uterus (from your fallopian tube) about three days after ovulation, or day 18 of a 28-day cycle. The “implantation window”, when it’s easiest for the egg to implant, lasts for about 4 days: days 6-10 after ovulation.

How does the egg stick to my uterus?

While the uterine lining thickens, its texture also changes. Estrogen will stimulate the growth of special cells called “decidual cells”. They make your lining more sponge-like, to allow the egg to implant easily.

The placenta will later develop here too, to send nutrients and oxygen to the fertilized egg as it develops into a baby.