First Trimester of Pregnancy

The first trimester is the time when the initial and most important part of your baby’s development takes place [1]. Although, it is pretty normal for half of the trimester to pass by before the mother realizes she is carrying, especially in case of an unplanned pregnancy.

How long is the first trimester

It is counted from the first day of your last menstrual cycle, continuing to the 12th or 13th week of pregnancy, about the first three months [2, 3].

First trimester weeks

Month of Pregnancy Week by Week
First month 1-2, 3, 4
Second month 5, 6, 7, 8
Third month 9, 10, 11, 12, 13

Conception and fertilization

Following the ovulation and fertilization, the egg travels through the fallopian tube, while dividing into multiple identical cells to finally form the blastocyst that attaches itself to the uterine wall (implantation), where it undergoes further development to grow into the embryo, fetus and finally into a healthy baby [4, 5].

First Trimester Baby Development

First Trimester Baby Development

What are the common first trimester symptoms

The symptoms of nausea, vomiting (morning sickness), increased urination, extreme tiredness and fatigue, along with breast tenderness are the first signs to indicate a pregnancy [6]. Many women start experiencing these symptoms even before a missed period or a positive home pregnancy test [7].

As your pregnancy advances, you may start getting the other common signs of mood swing, headache, cramps, constipation, indigestion, bloating, food cravings or aversions [8] and an incredibly strong sense of smell that often contributes to worsening the nausea [9]. In some cases, women may experience diarrhea instead of constipation. Although it is quite normal, consider calling your doctor if the diarrhea refuses to go away even after 3-4 days. The increasing estrogen and progesterone hormone levels are responsible for most of these symptoms, including the mood swings and emotional breakdowns [10].

It is advisable to contact your physician in case you have any of the alarming signs such as severe lower back pain, bleeding, passing blood clots and extreme nausea as these might indicate a miscarriage [11].

How much weight should you gain in the first trimester

Gaining weight is not a serious concern yet, as your baby is just as big as a peach by the end of the first trimester [21]. The ideal weight gain during these three months is only around 3-4 pounds [8], so continue following a balanced, nutritious diet instead of starting to eat for two.

When to announce your pregnancy

Once you get that positive home pregnancy test, it is normal for you and your partner to be too excited to keep the news to yourselves. But, it is usually a good idea to wait for the first trimester to be over before you can break it to your family and friends. According to the American College of Obstetricians and Gynecologists, around 10% to 25% of pregnancies end in a miscarriage [12], while 75% to 80% of all miscarriages occur within the first three months [13, 14].

The chances go down dramatically to 1% to 5% after the 13th week [15, 16]. So, many women prefer to keep the news a secret before they can be sure of their baby’s well-being, to avoid the sad and awkward situation where they would need to explain to everyone that they have lost their pregnancy.

Find out some creative ways to announce your pregnancy to your family and friends.

First trimester prenatal check-up and screening

Call your family physician or obstetrician for your first prenatal visit, as soon as you find out about the pregnancy. The first check-up is not usually scheduled before the 8th week of pregnancy, unless there is a risk of complications, such as a history of failed pregnancy or a family history of some genetic disorder [17]. The visit involves checking on the growing embryo for any abnormalities as well as calculating your due date and examining your medical history [18].

Things to keep in mind

  • The fundamental development of your baby’s nervous system occurs in the first trimester [5]. So, make sure to take your prenatal vitamins and folic acid supplements as well as avoid alcohol, caffeine and any drugs [2]. This is the time when the risk of neural tube defects is the highest [19].
  • Practice some relaxation techniques and follow a regular exercise routine (after consulting your physician) to reduce the chances of certain late pregnancy symptoms, while preparing your body for labor and childbirth [20].
  • Although flying during the first trimester is usually considered safe, it is recommended to consult your doctor once before making travel plans, especially in case of a high risk pregnancy [22].


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