Baby's Development - First Trimester

An average pregnancy lasts 40 weeks, and Pregnancy is commonly divided into thirds, or Trimesters, each lasting approximately 13 weeks. Your doctor and/or nurse-midwife will track pregnancy-progress in terms of weeks, beginning with the first day of your last menstrual period. During the 10-weeks after the first day of your last menstrual period (12 -13 weeks after fertilization), your developing baby is called an embryo. All the major organs are formed during this period, but they are incompletely developed. It is during the first Trimester when an embryo is most vulnerable to environmental toxins, and in which most miscarriages occur. Once the embryonic period is complete, your baby is now a Fetus.

Below are the highlights of key physiological developments you may expect, week-by-week:

Week 1

This is the week of Mom's last menstrual period. Calculated later by her doctor, the first day of bleeding is considered to be the official date of pregnancy.

Week 2

The uterus is made up of a muscular "myometrium", and a thin "endometrium" that lines the inside surface of the uterine cavity. During the usual menstrual cycle the endometrium grows into a velvety, blood-rich lining under the effects of progesterone. This thickened lining will provide a hospitable place into which the blastocyst can attach and implant. If the woman does not become pregnant the thickened endometrium is sloughed each month producing the typical menstrual flow. When the woman becomes pregnant, the endometruim into which the blastocyst implants first nourishes the blastocyst, and then later becomes part of the placenta, the organ that joins the fetus to the uterus. The endometrium that lines the uterine cavity that doesn't become part of the placenta remains in the "premenstrual" state until the end of the pregnanacy and is sloughed during delivery (and for possibly many days afterward) just like it is during the woman's menstrual period.

Week 3

  • On approximately the 14th day, one of the eggs travels into the fallopian tube, a process commonly known as ovulation.
  • During the next 24 hours, if one sperm (out of the 350 million in the average ejaculate) travels the entire distance (from Vagina through the uterus and into the fallopian tube) to penetrate the egg, fertilization has occurred.
  • In the uterus, the fertilized egg immediately begins cell division and floats down toward the uterine wall, where it embeds itself. At this stage the baby's gender is determined by the genes of the father.

Week 4

  • Usually, a woman will miss her period by the end of this week.
  • At this phase of your pregnancy, your embryo consists of two layers of cells, the epiblast and the hypoblast. These will eventually develop into all of your baby's organs and body parts. For the mother, the amnion and the yolk-sac develop at this time. The amnion (filled with amniotic fluid) surrounds and protects the growing embryo, while the yolk-sac helps to nourish until the placenta takes over that role.
  • Now, the baby has a home for the next 9 months! Here it is kept in safety, at a consistent temperature, and with plenty of room to grow without disturbing Mom's vital organs.
  • Mom begins to experience physical signs and symptoms of pregnancy.

Week 5

  • The amazing placenta has the ability to reach out and tap the blood supply in order to bring necessary nourishment to the baby through a long, jelly-like rope called the umbilical cord. Utilizing three blood vessels, the umbilical cord is also used to take away the baby's waste material. The umbilical cord has one artery delivering blood from the baby's heart to the placenta and two veins returning blood from the placenta to the heart.
  • Mom has missed her period by now, and a home or laboratory test will verify her suspicions. Most women schedule their first appointment with their OB/GYN at this time.
  • The next 5 weeks are critical to the development of the embryo.

Week 6

  • The embryo is now about 0.5 inches in length, weighing less than a half-ounce.
  • Tiny limb-buds are growing into arms and legs, and the embryo now has a beating heart.
  • The neural tube, which becomes the brain and spinal cord, is forming.
  • The liver, kidneys and other major organs begin their development.
  • Surrounding the embryo is the amniotic sac, which contains fluid to cushion the baby against possible injury. Amniotic fluid is a viscous substance that is easy to move around in, and also helps to maintain just the right temperature.
  • Mom may begin to experience fatigue, soreness of breasts and nausea.

Week 7

  • Approximately the size of a raspberry, the embryo's head is large, and dark spots have appeared which ultimately become the eyes and nose.
  • Eyelids, fingers, toes and muscles begin developing, and the neural tube has now closed.

Week 8

  • Your uterus is now about the size of a lemon.
  • The embryo now assumes its technical name, fetus. This Latin word translates to "young one".
  • Ears are now forming, and webbed fingers and toes have developed.
  • The beginnings of all necessary internal and external structures are present.

Week 9

  • Still positioned low within the pelvis, the uterus is now about the size of a tennis ball. It presses against the bladder, causing the necessity to urinate more frequently.
  • Although you cannot feel it (yet), the fetus is constantly in motion. Mom may notice that her bras no longer fit.

Week 10

  • Utilizing Doppler Ultrasound, the heartbeat may be heard by now (or definitely by the 11th week).
  • The fetus now has a large head and small body, looking almost like a shrimp.
  • The genitals have begun to form.

Week 11

  • Vital organs are developing, and tiny fingernails and hairs are forming.
  • Although the fetus still weighs less than 1-ounce, the doctor should be able to hear a rapid heartbeat.

Week 12

  • Although Mom will not feel these movements for quite some time, the fetus now begins to bend and stretch, moving its arms and legs, making fists, opening hands and lifting its head.
  • The eyelids are completely developed, and tooth-bud are forming along with the vocal cords.
  • The kidneys have formed and the fetus begins to pass urine into the amniotic fluid.
  • The fetus is now up to 4 inches in length and weighs a little more than 1 ounce.
  • The chance of miscarriage has been greatly reduced.