What is implantation? A short guide

The “two-week wait” between your carefully calculated ovulation and taking a pregnancy test can be very stressful, so you may be wondering: how does implantation occur? What conditions make implantation possible? And how might you know if it has occurred? Let’s take a look.

What is implantation?

Implantation is the attachment of a fertilized egg to the wall of the uterus and typically occurs between 6 and 12 days after ovulation, with most cases happening around day 9. At the point where the fertilized egg enters the uterus, it’s known as a blastocyst — a round collection of stem cells in a fluid with an outer layer that eventually forms the placenta. This outer layer is essential in this process because it is what adheres to and merges with the endometrium or uterine lining.

What is the endometrium, and what does the endometrium do?

The uterine lining -- also known as the endometrium -- is composed of two layers:

  • The basal layer never sheds. It is the part of the endometrium that helps form the other layer.
  • The functional layer builds up after the previous menstrual cycle ends, forming a perfect environment for a fertilized egg. If the egg is not fertilized, the functional layer will shed during the next menstrual cycle.

When an egg is fertilized, it implants in the uterine lining. This usually occurs between 6 and 12 days after ovulation and about 5 to 6 days after fertilization.

Once implanted, the embryo secretes hormones, including human chorionic gonadotropin (hCG), which signals to the mother’s body that she is pregnant. This halts the menstrual cycle and prevents the functional layer of the endometrium from being shed.

From this point on, the uterine lining will turn into the placenta, which provides oxygen and nutrients to the baby.

Even when there’s no implanted egg to sustain, the uterine lining serves a critical function. The endometrium prevents adhesions of the underlying myometrium layer, which contains the muscle tissue that causes contractions. If adhesions form, it’s possible for the uterus to collapse, preventing normal menstruation and pregnancy.

What role does the endometrium play in implantation?

Studies show that cycles with thicker uterine linings result in successful pregnancies more often. In most successful implantations, the arrival of the egg coincides with peak levels of luteinizing hormone (LH), which primes the endometrium to accept the embryo.

In addition to LH, a chemical called trypsin, produced by the embryo signals the uterine lining to prepare for implantation. A favorable endometrium is thick and exhibits a triple-line pattern under ultrasound imaging.

Unfortunately, even if an egg is successfully fertilized, it is possible for the embryo to fail to implant in the uterine lining. In two out of three cases, this is due to inadequate uterine receptivity. Many factors can contribute to uterine receptivity, including hormones and a type of proteins called cytokines, which allow cells to send and receive signals. Some medications, including progestins and progesterone, can increase the chances of successful implantation after repeated failure.

What can cause implantation to fail?

Unfortunately, various conditions can hamper the proper implantation of a blastocyst. In some cases, genetic disorders in the developing embryo disrupt the trypsin signal and cause a stress reaction that forces the uterus to reject the blastocyst. Immunological disorders may cause the mother’s body to attack the implanting embryo. Ultimately, this is a delicate process, and roughly half of all fertilized eggs fail to successfully implant.

Signs of Successful Implantation

If the embryo successfully implants, you can expect to experience a number of signs or symptoms. Unfortunately, the earliest signs of success can appear like the normal signs of a period: cramps, headaches, fatigue, and bloating. In 20% to 30% of women, implantation bleeding occurs, similar to what they experience during a period.

However, if it's implantation and not  your period, additional symptoms will follow:

  • Your breasts may feel sore, tight, or tender.
  • If you continue tracking your basal body temperature after ovulation, you may notice that your average temperature has increased.
  • This rise in temperature, combined with fatigue and other symptoms, may make you think you have the flu.
  • Within a week, you may feel more frequent urges to urinate. This is because increased blood flow to the uterus has put pressure on your bladder.

Of course, the only way to tell if you have had a successful implantation is to take a pregnancy test. If you have any questions, schedule an appointment with your doctor.

Read More About Implantation