First Trimester

Most people think the gestation period, or the time it takes for a fetus to fully develop, is nine months. Actually, it’s 280 days, or 40 weeks. Your pregnancy is measured in weeks and is divided into three phases called the First, Second and Third Trimesters.

Morning Sickness?

Eighty percent of all pregnant women feel queasy or have morning sickness during their first trimester, and it usually subsides after the first three months of pregnancy.1,2,3 No one knows what causes nausea during pregnancy, although it’s probably a combination of the many physical changes taking place in your body. Some vitamins and foods may make it worse. Even if you do experience morning sickness, it’s essential that your baby receives the nutrients it needs to grow healthy and strong. 

It’s important that you maintain a healthy diet during your pregnancy, but eating right will not provide all the nutrients your body needs. Your body needs twice the amount of some nutrients in the first trimester, so it’s a good idea to take prenatal vitamins. Ask your doctor about the best prenatal vitamin with gentle ingredients to optimize nutrition for both you and your baby. 

First Trimester Timeline

  • 1 to 3 Months: The embryo undergoes rapid growth and development. The bones, muscles, lungs, kidney and liver begin to grow, including an early spinal cord (neural tube). During the third and fourth week, the heart begins beating.
  • Week 8:  The major joints, legs and arms have developed and the embryo begins to move. The embryo is now a fetus.
  • Weeks 8 to 12: Circulation begins in fetal blood cells and eyelids; ears and teeth buds are forming.  

During your first trimester you may begin to notice maternal symptoms — your hormonal changes may make your breasts tender, sore, or feel enlarged or heavier; you may suffer from morning sickness during the day or night; weight gain is possible; and you may experience increased vaginal discharge.

New Developments

Although it may not be visible to others, your body is undergoing a number of changes. 1

  • Uterus — By the end of your first trimester, your uterus will increase from the size of a plum to the size of a grapefruit. Inside your uterus, the fetus is surrounded by a placenta and receives all nutrients through the umbilical cord.
     
  • Stomach — The baby bump may not be showing yet, but it’s normal for women to notice that their clothes may fit a little tighter and they may feel a little bloated. If you are one of the many women whose first sign of pregnancy is morning sickness, it may continue until about week 14.
     
  • Breasts — Until the 8th or 10th week, your milk glands are multiplying and fat layers are growing to accommodate the extra calories your baby needs. Your breasts may feel larger and sore to the touch. At the end of your first trimester, breast growth begins to slow down.

Normal physical symptoms you may experience during your first trimester1:

  • Constipation
  • Fatigue
  • Frequent urination
  • Food aversions
  • Food cravings
  • Heartburn, indigestion, excess gas and bloating
  • Increased appetite
  • Infrequent faintness or dizziness
  • Occasional headaches

Function of the Placenta During Pregnancy

The placenta is an organ that acts as the exchange and filter center for you and your baby. Throughout your pregnancy, the placenta continues to grow and at birth it may weigh about a pound. The placenta consists of “villi,” numerous small projections which extend into the blood vessels of the uterine wall. Blood flows through the spaces surrounding the villi. Antibodies, nutrients and oxygen are exchanged with the blood of the fetus through a thin membrane.


  1. Tabers Cyclopedic Medical dictionary. 22nd ed. in: Venes D, editor. Philadelphia: F.A. Davis Company; c2005
  2. Gadsby R, Barnie-Adshead AM, Jagger C. A prospective study of nausea and vomiting during pregnancy. Br J Gen Pract. 1993 Jun;43(371):245-8
  3. Quinta JD, Hill DA. Nausea and vomiting of pregnancy. Am Fam Physician. 2003 Jul 1;68(1):121-8
  4. Sahakian V, Rouse D, Sipes S, Rose N, Niebyl J. Vitamin B6 is effective therapy for nausea and vomiting of pregnancy: a randomized, double-blind placebo-controlled study. Obstet Gynecol 1991 Jul,78(1):33-6.