New Health Guide

Stages and Length of Human Gestation

Nov 22, 2017

Pregnancy, or human gestation, begins at conception when the sperm from the man fertilizes the egg, or ovum, of the woman. It continues until the child is born. The “normal” gestation period is about 40 weeks, but may range from 37 to 42 weeks and still be considered a normal pregnancy. It is most often the case that a woman does not know exactly when fertilization of the egg occurred, so the obstetrician or midwife will calculate the expected period based on the first day of the woman’s last normal menstrual period.



First let's watch the video to get a general idea of what happens in the 9-month pregnancy period:


Gestation Period for Humans

When healthcare workers talk about this, they normally refer to nine months divided into three 3-month long “trimesters”.

Stages of Human Gestation

First Trimester

The first three-month trimester is generally calculated as starting on the first day of the last period and runs through the 13th week of the pregnancy. During this trimester, the fetus implants into the womb of the woman. A woman may not realize she is pregnant until she misses a period or begins to have nausea, vomiting and fatigue that are commonly known as “morning sickness”. This is an important time of the gestation period, because it is the trimester during which the expectant mother’s medication and alcohol use can have adverse effects on the fetus. The first trimester is also the time when miscarriages, or loss of the pregnancy, are most common.

Second Trimester

The second three-month trimester starts in the 14th week of the pregnancy and runs through the 27th week. During the 2nd trimester, most women report that they feel great (morning sickness gone and mother-to-be more energetic) and the baby begins to grow rapidly. By about the 20th week of the gestation period, most women will begin to feel movement of the fetus that is often described as feeling “like butterflies flapping their wings”. Many healthcare providers will offer an ultrasound during this trimester to determine if there are any fetal abnormalities that can be detected. It is during this ultrasound that many prospective parents find out the gender of the baby.

Third Trimester

The third three-month trimester starts in the 28th week and runs through the birth of the child. During this period, the fetus increases in size and stretches the uterus and abdomen of the woman. In the first part of the 3rd trimester, the fetus will be at its most active and movement will be very obvious. As the child fills up the available space, movement will become less but stronger and the woman will become more uncomfortable. It is during the last 2-3 weeks that contractions begin that will force the baby down into the birth canal in preparation for birth.

Typically, delivery will happen vaginally after up to 24 hours of labor. Occasionally, the birth must be through a Caesarian section – a surgical procedure during which the surgeon cuts open the woman’s abdomen and womb to deliver the child.

If labor and delivery have not occurred by the 42nd week of the gestation period, a physician may recommend “inducing” labor using medications. This process will chemically induce a woman to start the labor process.

Length of Gestation Period for Humans Can Vary by as Much as Five Weeks

According to research published in Human Reproduction, It can vary by as much as five weeks and still be considered a “normal” pregnancy. The statistics show that only about 4 women out of every 100 pregnancies deliver on their due date. Further, only 70 out of 100 will deliver within 10 days of their estimated delivery date.

In the study reported in the journal, the researchers were able to determine the exact moment of ovulation and implantation in the womb. From these dates, they were able to determine that the time from ovulation to birth is 268 days. Even with that knowledge, the researchers found that there is STILL a potential five week variability in the gestation period.

In the 130 pregnancies that were followed in the study, the women kept detailed diaries of their efforts to become pregnant. In addition, they each submitted first-morning urine samples to determine the exact day of ovulation and the exact day of fetal implantation. Changes in estrogen, progesterone and hCG levels in the urine were the markers used to determine the two dates. Of the 130 pregnancies that began in the study, the researchers were able to follow 125 “normal” pregnancies. Factors that seemed to influence the length of the gestation period included:

  • For pregnancies with longer time from conception to implantation, the gestational period was also longer.
  • Pregnancies with a late increase in the progesterone level resulted in a shorter gestational level.
  • Older women had longer gestational periods.
  • Higher weight of the mother predicted a longer gestational period for her child.
  • Women with previous history of longer gestational periods tended to have a longer pregnancy in the study, which would indicate that most women may be consistent in the length of their own gestational period.