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).

The science of your uterus

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.

Fertility challenges

Because your uterus will be the fertilized egg’s first “home”, fertility challenges can occur if your uterus has issues. Specifically, it should have a good shape and a well-functioning lining.

Polyps or fibroids

These guys are two types of benign growths inside the uterus. They can make it harder for the fertilized egg to implant or develop. Polyps and fibroids are fairly common – if they’re small enough, they may not even create problems. Common symptoms are very heavy periods and mid-cycle spotting. If this sounds like you, consult your family doctor or gynecologist to get checked out.

Shape problems

Arcuate subseptate is a fancy way of saying that that top of your uterus is in a concave shape (dipping down in the centre). This doesn’t usually cause difficulties conceiving or implanting, but it can create challenges in carrying the pregnancy to full-term. Unless you go to a clinic to have an ultrasound, you can’t know for certain if your uterus is a normal shape. Remember: in most cases, your uterus will be fine.

Lining problems

Adenomyosis and endometriosis are two conditions that could affect the condition of your uterine lining. Common symptoms include painful sex, cramping, excessive or heavy menstruation and unexplained spotting throughout the cycle.

With adenomyosis, your uterine lining is unusually thick. That restricts the blood flow, which makes it harder for the embryo to implant. This usually happens later in a your childbearing years, after you’ve already delivered a baby. Adenomyosis can be diagnosed with an MRI or ultrasound.

Endometriosis is a condition where tissue in your pelvic organ is “fooled” into thinking it’s uterine lining – even though it isn’t. When your hormones tell the uterus to thicken and grow its lining, this other tissue starts thickening and growing too! This can happen all over the pelvic region, including the outside of your uterus.

Although some endometriosis cases are relatively superficial and minor, it can be pretty serious: it can cause internal bleeding, disrupt your pregnancy, and create scar tissue that could damage or block your ovaries and fallopian tubes. Because it responds to female hormones, symptoms such as cramping and spotting might increase as you approach menstruation.

If you’re concerned that you may have endometriosis, your doctor may want to perform a test called a laparoscopy to diagnose you.


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