3 Weeks of Pregnancy | First Trimester | Pre-Embryonic Stage

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Pregnancy is measured using “gestational age.” Gestational age starts on the first of one’s last menstrual period (LMP) since it is difficult to tell the exact day of conception [1].

At one or two weeks of pregnancy, a person is not technically pregnant since conception has not yet occurred. Ovulation generally occurs about three weeks after one’s LMP. Sperm must then fertilize the egg for pregnancy to happen.

At three weeks of pregnancy, the sperm and egg have joined in the fallopian tubes. From the time of fertilization, a human pregnancy generally lasts 266 days which is 38 weeks or 8.75 months [1].

The fertilized egg is now known as a zygote [1]. As the cells within the zygote start dividing, it becomes a mass known as a morula [1].

About four days after fertilization, the morula enters the uterus and develops into a blastocyst [1,2]. The inner cells of the blastocyst will develop into the baby, and the outer layer will develop into the yolk sac and placenta [2].

By the end of this week, the blastocyst will start to implant into the uterine endometrium (uterine lining). This process is called implantation and is successful 25–50% of the time [3]. If a blastocyst does not implant, one’s body will shed it during menstruation. If a blastocyst does implant, one’s menstrual cycles will temporarily cease.

When implantation occurs outside of the uterus, it is called an ectopic pregnancy. One in 50 pregnancies is ectopic, and most of these ectopic pregnancies (95%) are tubal pregnancies [4,5]. A tubal pregnancy occurs in the fallopian tubes rather than in the uterus.

1. Moore KL, Persaud TVN, Torchia MG. The Developing Human: Clinically Oriented Embryology. Philadelphia: Elsevier; 2016.
2. Harms R, Wick M, editors. Mayo Clinic Guide to a Healthy Pregnancy. Boston: Da Capo Press; 2011.
3. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al. 28.2 Embryonic Development. Anatomy & Physiology. Houston: OpenStax; 2013.
4. Cunningham FG, Leveno KJ, Bloom SL, Hauth JC, Rouse DJ, Spong CY, editors. Ectopic Pregnancy. 24th ed. Williams Obstetrics. 24th ed. Mc Graw Hill; 2013.
5. Tenore JL. Ectopic pregnancy. Am Fam Physician. 2000;61: 1080–1088.

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Produced and written by: Samantha Lattof, PhD MSc
Medically reviewed by: Ayodeji Olelakan Abere, MD MBA MHA
Animation by: SciePro / Pond5
Music by: audiomarket / Pond5
Narration by: Ava at Wellsaid


Disclaimer: This content including advice provides generic information only. It is in no way a substitute for a qualified medical opinion. Always consult a specialist or your own doctor for more information.

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