First Trimester

The first trimester is the most crucial period in your baby’s development. This is when all the organ systems develop, and it is also during the first trimester that most miscarriages happen.

There is some diversity of medical opinion about exactly when the first trimester ends. The Office on Women’s Health at the US Department of Health and Human Services defines the first trimester as weeks 1–12, and PregnancyCorner follows their lead. However, the American Pregnancy Association and the UCSF Medical Center at the University of California, San Francisco both consider the first trimester to run from week 1 to week 13. Other experts extend it until the end of week 13, with the second trimester starting at 14 weeks exactly.

First Trimester Pregnancy Symptoms

During the first trimester of your pregnancy, you are likely to experience sensitivity to odors, nausea with or without vomiting (or “morning sickness”), breast tenderness, severe fatigue, occasional headaches, and possibly heartburn. You will feel the need to urinate frequently due to the pressure exerted on your bladder by your growing uterus—yet, paradoxically, you may also experience constipation.

During the first trimester you may also experience unusual food cravings or food aversions, and your friends and family may notice that you are prone to mood swings.

Because pregnancy causes blood vessel dilation and a drop in blood pressure, you may have bouts of lightheadedness or dizziness. Weight gain is, of course, quite common, but a minor degree of weight loss is also not unusual due to the effects of morning sickness, and not necessarily anything to be alarmed about (although you should report it to your doctor).

Progesterone levels rise during early pregnancy, and this may induce sleepiness or constant fatigue—but some women report increased levels of energy, although this is much more common in the second trimester. Progesterone also can lead to bloating and water retention, which are common symptoms throughout pregnancy.

By the End of the First Trimester

The embryonic period has ended and the fetal period has begun. Your fetus will have grown to about 3 inches in length and should weigh about an ounce. The baby can now make a fist, and the buds that will become baby teeth may have begun to appear.

Read more about your week:

This page was last updated on 06/2017
What do you need help with?