Stephen J. Schueler, M.D.

Pregnancy Physiology

Stages of a normal pregnancy include:

  • Ovulation
  • Fertilization
  • Implantation
  • Growth
  • Delivery

Doctors divide a normal pregnancy into three equal divisions called trimesters. The first trimester of pregnancy extends from conception to 12 weeks after the last period, or slightly less than 3 months. The second trimester of pregnancy extends from the 13th week until the 27th week. The third trimester of pregnancy extends from 28 weeks after the last normal period to birth.

Pregnancy Step 1 Ovulation

The female reproductive organs include the vagina, cervix, uterus, fallopian tubes and ovaries. The lining of the uterus is called the endometrium.

Before ovulation, the endometrium grows and becomes thick: the endometrial tissue and blood vessels support the fertilized egg when it enters the uterus.

During ovulation, an egg is released from one of the ovaries and enters the fallopian tube. If the egg is fertilized as it passes through the fallopian tube, it attaches to the endometrium on the inside of the uterus. The endometrium continues to support the fertilized egg as it grows.

If the egg is not fertilized, the egg does not attach to the endometrium and the endometrium breaks down. The uterus sheds the endometrium, which causes the bleeding associated with the menstrual period.

Pregnancy Step 2 Fertilization

Our genetic material is carried in structures known as chromosomes. Our chromosomes provide the specific "blueprint" for producing a human being. The egg and the sperm each contribute one-half of the genetic material, or chromosomes, required to produce a human being.

During fertilization, millions of sperm will try to enter the egg in an attempt to share its genetic material. If one sperm is successful, the resulting organism will have a complete set of 46 chromosomes. Without fertilization, the egg does not have enough genetic material to produce a human being and will eventually die.

Conception is the moment when a single sperm successfully passes inside the egg and completes the necessary number of chromosomes necessary for life. At this point, no other sperm can enter the egg. Fertilization and conception usually occurs when the egg is making the trip through the fallopian tube from the ovary to the uterus.

Conception triggers a rapid process of repeated cell divisions and growth. One cell becomes two, two become four, and so on. Several days later it will arrive at the uterus. At this time it is only a collection of cells, known as a blastocyst. A blastocyst is so small that a microscope is needed to see it.

Pregnancy Step 3 Implantation

About one week after conception, the fertilized egg, or blastocyst, is ready to implant, or attach to the wall of the uterus.

The blastocyst must become implanted in the wall of the uterus if the pregnancy is going to continue. Implantation occurs in the inner lining of the uterus, called the endometrium. The endometrium is rich in blood vessels and nourishes the fetus as it grows during the 9-month pregnancy.

Pregnancy Step 4 Embryo Growth

After implantation, the blastocyst begins to turn into an embryo, which is a more advanced stage of development. The process of becoming an embryo is complete when the amniotic sac and the placenta form. The fetus develops inside the amniotic sac, floating in amniotic fluid. The placenta is attached to the wall of the uterus. It delivers oxygen and nutrients from the mother's bloodstream to the fetus, during the pregnancy.

Pregnancy Step 5 Hormones

Pregnancy is a process that is controlled by a complex interaction between female sex hormones. The tiny embryo produces a female sex hormone called human chorionic gonadotropin. A standard pregnancy test detects this hormone in the urine or bloodstream.

Human chorionic gonadotropin signals the ovaries to stop ovulating and prevents the uterus from shedding the endometrium. This is why lack of menstruation is one sign of pregnancy. It also stimulates the placenta to produce another hormone, called progesterone. This hormone prevents the uterus from contracting, which would disturb the pregnancy. These hormones are also necessary in order to protect the uterus so it can provide nutrition to the developing fetus throughout the term of the pregnancy.

Pregnancy Step 6 First Trimester

The first trimester of pregnancy extends from conception, which occurs about 2 weeks after the last normal menstrual period, to 12 weeks after the last period: slightly less than 3 months. At the end of the first trimester, a fetus grows to about three and one-half inches long, and weighs about a half an ounce.

A fetus' heart starts beating in the middle of the first trimester. The kidneys, liver, and lungs develop during this time. The arms, hands, fingers, legs, feet, and toes also develop during this time.

Pregnancy Step 7 Second Trimester

The second trimester of pregnancy extends from the 13th week until the 27th week. Early in the trimester, the fetus grows to just over 2 pounds. At the end of the second trimester, a fetus is about 12 inches long. At the end of the 16th week, the top of the uterus will be halfway between the mother's pubic bone and belly button.

By the end of the 27th week, the top of the uterus will be about 1 inch above the belly button. Through the second trimester, the fetus begins to move more and its limbs are fully formed. By the end of this trimester, the fetus can open his or her eyes and now has eyelashes. During this time it is common for the fetus to begin sucking a thumb.

Pregnancy Step 8 Third Trimester

The third trimester of pregnancy extends from 28 weeks after the last normal period to birth. Childbirth usually occurs between the 38th and 42nd weeks of the pregnancy. During the third trimester, a baby continues to grow larger and the organs mature.

The mother will probably notice increase movement from the baby early in the third trimester. As the baby gets larger during the last 2 months, it may seem like movement is less. This is because there is simply less room to move.

During the third trimester, a baby's weight will go from about 2 pounds to about 7 pounds. The baby will be 13 inches long in the beginning of the third trimester and be around 19 inches at the end of the trimester. By the end of the third trimester, the bones of the skull are becoming hard, but the bones in the limbs are still growing rapidly. As the pregnancy draws to an end, a baby usually moves into a head-down position. This is the best position for the baby to deliver when labor occurs.

Pregnancy Step 9 Labor and Delivery

Labor is the process that results in the delivery of the infant.

Although most women recognize when "true labor" begins, nobody can predict when it will start, or how long it will last. First time deliveries are particularly difficult to predict. The labor tends to last longer and progress more slowly compared to women who have experienced childbirth in the past.

Doctors divide labor into three stages.

First Stage
The first stage of labor occurs when the cervix dilates. This is necessary since the opening to the cervix must become larger to accommodate the passage of the infant. First-time mothers usually have about 12 hours of first-stage labor. Women who have delivered children before average about 6 hours in first-stage labor. Your doctor will perform periodic checks of your cervix to see how much you are dilated. You will not be able to deliver your baby until the cervix is completely dilated (usually 10 centimeters or more).

Second Stage
The second stage of labor is the actual birth itself. It usually begins after the membranes have ruptured, at which time you will leak a large amount of clear fluid from your vagina. This is the amniotic fluid that the baby floated within during the prior 9 months of development. The second stage of labor is when the baby is pushed from the uterus due to the strong contractions of the uterine muscles. The length of the second stage is also variable. It may last as little as 3 to 5 minutes or take as long as 2 or 3 hours. Your doctor will be closely monitoring the status of the baby through the second stage of labor.

Third Stage
After you deliver your baby the third stage of labor begins. This is the time when you deliver the afterbirth, or placenta. After delivery, the placenta will loosen its grip on the wall of the uterus and be pushed out of the uterus with the remaining contractions. Most women don't notice this event, since they are so preoccupied with the delivery of their child.

Last Updated: Jul 7, 2009 References
Authors: Stephen J. Schueler, MD; John H. Beckett, MD; D. Scott Gettings, MD
Copyright DSHI Systems, Inc. Powered by: FreeMD - Your Virtual Doctor

PubMed Pregnancy References
  1. Blenning CE, Paladine H. An approach to the postpartum office visit. Am Fam Physician. 2005 Dec 15;72(12):2491-6. [16370405]
  2. Condous GS, Arulkumaran S. Medical and conservative surgical management of postpartum hemorrhage. J Obstet Gynaecol Can. 2003 Nov;25(11):931-6. [14608443]
  3. Magann EF, Evans S, Hutchinson M, Collins R, Lanneau G, Morrison JC. Postpartum hemorrhage after cesarean delivery: an analysis of risk factors. South Med J. 2005 Jul;98(7):681-5. [16108235]
  4. Malamitsi-Puchner A, Boutsikou T. Adolescent pregnancy and perinatal outcome. Pediatr Endocrinol Rev. 2006 Jan;3 Suppl 1:170-1. [16641854]
  5. Nanda K, Peloggia A, Grimes D, Lopez L, Nanda G. Expectant care versus surgical treatment for miscarriage. Cochrane Database Syst Rev. 2006 Apr 19;(2):CD003518. [16625583]
  6. Ozkaya O, Sezik M, et al. Placebo-controlled randomized comparison of vaginal with rectal misoprostol in the prevention of postpartum hemorrhage. J Obstet Gynaecol Res. 2005 Oct;31(5):389-93. [16176505]
  7. Tierney JP, Welsh J, Owen P; Effective Gynaecology in Glasgow Group. Management of early pregnancy loss--a complete audit cycle. J Obstet Gynaecol. 2006 Apr;26(3):229-32. [16736559]
  8. Vitzthum VJ, Spielvogel H, Thornburg J, West B. A prospective study of early pregnancy loss in humans. Fertil Steril. 2006 Aug;86(2):373-9. [16806213]
FreeMD is provided for information purposes only and should not be used as a substitute for evaluation and treatment by a physician. Please review our terms of use.