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Pregnancy can be of the most interesting and exciting times in a woman's life. A pregnancy is classified into three trimesters, and though each one is completely different, the first trimester can be the most difficult and also the most interesting.

During sexual intercourse a male will release between 150-400 million sperm upon ejaculation.  If conditions within the cervical fluid are suitable, some sperm can survive for a period of up to 5 days.  Approximately 200 sperm will actually make it through the uterus and into the right fallopian tube, eventually encountering the ovum released by an ovulating female.  

At some point within the next 24-48 hours after ovulation, only one sperm or in some cases two, will have enough strength to make it to the egg and break through the outer layers of the ovum, and result in conception.  At this point, the egg is fertilized and pregnancy results.

The fertilized egg spends the next 3-7 days traveling down through the fallopian tube on a journey to implant itself into the uterus.  When the egg arrives at the uterus, it will float freely for a period of several days and finally implants into the wall of the uterus, thereby establishing the pregnancy.

What Does the First Trimester Mean for Mother and Infant (Embryo)

During the first trimester, many women are unaware they are pregnant, though the zygote is already undergoing many rapid changes. Once the pregnancy becomes firmly established, both mother and the developing zygote experience a great many changes in a relatively short period of time.

At this point, a woman may first begin to experience some side effects that indicate she is pregnant.  The main symptoms of pregnancy include breast tenderness, fatigue, nausea, food cravings, a normal vaginal discharge, mood swings and dizziness.  A woman can experience all, some or none of these symptoms, every person is different.

Fetal Development: 1-13 Weeks

The first 13-weeks of pregnancy form the first trimester, with each passing week new developments unfold for both the mother and the developing embryo.  To make sure the mother and embryo are both healthy, it is important to monitor maternal health during each stage of pregnancy.  The following pregnancy week-by-week information will give a general idea of what a woman can expect during the first 13-weeks of pregnancy:

  • Week 1:  embryo has not yet undergone conception and menstrual period begins. Most women do not experience signs of pregnancy until 10-14 days after ovulation.
  • Week 2:  maternal uterus is developing preliminary blood supply in anticipation of fertilization.
  • Week 3: sperm and egg fuse together, resulting in conception.  The growing zygote is the size of a sewing pin head, or approximately .006 inches long. The chromosomes of the father determine the gender of an infant.
  • Week 4: fused egg or blastocyst, becomes embedded in the uterine wall and keeps undergoing cell division. Embryo measures ½ to 1 millimeter long., the embryonic stage now begins. 
  • Week 5: developing embryo still a bundle of cells, rudimentary cells forming the nerves, brain and spinal cord begin to organize.  Embryo is still about the same size, around one millimeter in length or slightly less.
  • Week 6:  becomes an embryo, is approximately one inch in length.  Eyes and ears begin to develop, blood is being pumped and first buds of limbs start to show up. Maybe the time for first prenatal visit and the doctor does what is called a “crown to rump” length measurement of the womans abdomen, embryo will be between 2-5 millimeters in length by the end of the week.
  • Week 7:  tiny arms, legs, eyes and nose have formed and the brain is developing. Embryo around 13 millimeters in length and the size of a small grape.
  • Week 8:  embryo now has heartbeat, mother may be able to undergo first ultrasound to check for fetal viability and development.  By the end of the week, embryo is approximately 14-20 millimeters in length or about the size of a snow pea.
  • Week 9:  the embryo has developed bone and cartilage, webbed feet and hands are forming. Mother may experience indigestion and flatulence, rest and proper diet are important for health of the mother and embryo.  Embryo is around 1-inch long and weighs less than an ounce.
  • Week 10:  embryo is almost fully formed, from the tiny limbs to the face, brain and heart.  Embryo has grown to 31-42 millimeters long and size of a lime or slightly bigger.
  • Week 11:  week of rapid growth and further development in the embryo, skin is paper thin, embryo can open and close fist, by end of the week embryo can fit into the palm of hand. Embryo is approximately 1.75-2.4 inches in length, weighs around .3 of an ounce and is the size of a plum.
  • Week 12:  limb development is almost complete, sexual organs have begun to develop, brain secretes hormones and nerve cells develop rapidly during this time.  Embryo is around 2-inches long and weighs about ½ ounce. May be able to detect a heartbeat using a fetal doppler.
  • Week 13:  marks the end of the 1st trimester, embryo is developing a digestive system.  Averages between 2-3 inches in length and weighs around .7 of an ounce.  Head is large, eyes begin to move closer together, intestines are moving further into the body and pancreas begins to manufacture insulin.

What the Mother Experiences During the 1st Trimester

During the first trimester a womans body is undergoing many rapid changes.  By the time a woman recognizes the symptoms of pregnancy, she has entered the first trimester.  Among the more common early signs of pregnancy, a woman may experience nausea, vomiting, swelling and tenderness of the breasts, mild weight gain, food cravings, fatigue, backaches, mood swings and stress.

While these symptoms are common, some women sail through the first trimester with little to any type of side effects.  Nausea and vomiting will more than likely stop by the end of the first trimester, though rare, it may continue throughout pregnancy.

When in the first trimester, most women schedule the first prenatal appointment.  The first visit is one of the most comprehensive a woman will go through in her pregnancy.  At the time of the appointment, an obstetrician will take an in-depth medical history, conduct a full physical examination, blood pressure check and order the woman to go to a laboratory for certain blood tests, which include:

  • Blood type, Rh factor (a protein substance present in the red blood cells of most people) and antibody screening
  • Complete blood count (CBC)
  • Rubella (German measles) immunity
  • Hepatitis B screening
  • Syphilis screening
  • HIV testing
  • Other blood tests are possible if necessary

Recommended Weight Gain For Women in the 1st Trimester

A healthy weight gain for the first trimester for a woman of normal weight is between 3-5 lbs.  For a woman that is underweight, a 5-6 lb., during the first trimester is advisable, this can also depend on how many pounds underweight the mother was before pregnancy and the advice of the physician.  If a woman was overweight before pregnancy, a weight gain of 1-2 lbs., is recommended during the first trimester and 1 lb., each week for the rest of the pregnancy. 

(Weight gain for twins or multiples will vary, a woman should consult with a physician for the recommended amount of weight gain required.)

General Facts About the 1st Trimester of Pregnancy

The first trimester of pregnancy brings about many different physical changes for a woman, though there are some general facts that most women are familiar with, which include the following:

Breast tenderness and enlargement:  the release of progesterone and estrogen is at higher than usual levels and as a result a woman may experience enlargement and tenderness in the breasts. The nipples may darken 1-2 shades, white bumps may appear, an increase in blood flow to the area may also bring out blue lines in the breast, which are actually blood vessels. 

Shortness of breath and fatigue:  during pregnancy the demands of a growing infant puts more stress on a female body, as a result fatigue is quite common.  Shortness of breath can be caused by the pregnancy hormone progesterone or as a result of an expanding uterus. With regular rest periods, healthy diet and light exercise, a pregnant woman can combat some of the symptoms of fatigue.

Irritability and mood swings:  irritability and mood swings are commonly attributed to fatigue and surging hormones.  A pregnant woman can help counteract the mood swings and irritability by getting enough rest and eating a healthy diet.

Morning sickness and nausea:  attributed to influx of hormones, stretching of the uterus and added pressure the fetus puts on the digestive tract.  The vomiting and nausea usually subside after the first trimester, however, a woman can alleviate the problem by eating smaller meals spread out through the day.  Avoiding fatty, greasy foods can also help reduce nausea and vomiting.

Frequent urination:  the rapid growth of the uterus results in pressure on internal organs including the bladder.  To avoid the problem make sure to lean forward during urination to help completely empty the bladder, thus reducing the likelihood of increased urination.

The End of the 1st Trimester

By the end of the first trimester, most of the bothersome side effects of early pregnancy have become a thing of the past, making it a period of welcomed relief for many women.  As the first trimester ends and the second one begins, a woman moves toward a more enjoyable period of pregnancy and starts to relax about the changes her body is going through. 

The risk of miscarriage has dropped significantly by the end of the first trimester, which allows many women to relax a bit and actually begin to enjoy the pregnancy.  As a woman enters what many consider the “golden period” second trimester, the reality of pregnancy has set in and the mystery and wonders of the next twenty-seven weeks are waiting to be discovered.