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Mark A Curran, M.D., F.A.C.O.G.

The actual embryo or fetal age (also known as conceptual age) is the time elapsed from fertilization of the egg  near the time of ovulation . However,  because most women do not know when ovulation occurred, but do know when their last period began, the time elapsed since the first day of the last normal menstrual period , the menstrual age,  is used to determine the age of a pregnancy. The menstrual age is also known as the gestational age.  Gestational age is conventionally expressed as completed weeks. Therefore, a 36 week, 6 day fetus is considered to be a 36 week fetus.  [25]

The calculator below can be used to estimate the gestational age of a fetus on a given date.
Enter the estimated due date, then enter the date to estimate the gestational age on and click the "Calculate" button.

ENTER The Estimated Due Date mm/dd/yyyy

ENTER the Date To Calculate the Gestational Age On mm/dd/yyyy

First Trimester Week
  1 2 3 4 5 6 7 8 9 10 11 12 13
Second Trimester Week
14 15 16 17 18 19 20 21 22 23 24 25 26 27
Third Trimester Week
28 29 30 31 32 33 34 35 36 37 38 39 40 41

Fetal Development Chart
 
Developmental stage Embryonic Stage Fetal Stage
Gestational  Age (weeks) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 20 40
Conceptual  Age  (weeks) 0 0 1 2 3 4 5 6 7 8 9 10 11 12 18 38
Developing Organ(s)  
Central Nervous System    
Heart    
Ear        
Eyes    
Limbs  
Lip  
Palate    
Teeth      
External genitals    

The red bars in the table  show the gestational age when different organ systems are most sensitive to major birth defects in that organ system. The pink bars show the gestational age when different organ systems are sensitive to functional defects and minor malformations.


First Trimester of Pregnancy [4]
 ( Less than14 weeks 0 days)

Weeks 1 and 2 of Pregnancy
During the first two weeks after the last menstrual period egg follicles mature in the ovaries under the stimulus of follicle-stimulating hormone (FSH) a hormone secreted by the pituitary gland in the brain. High levels of the hormone estradiol, produced by the developing egg follicle,  cause secretion of luteinizing hormone (LH) ,yet another hormone from the pituitary gland. LH causes release of the egg from its follicle (ovulation)

For women with 28-day cycles, ovulation usually occurs on days 13 to 15.

 

Gestational Age 3 weeks  . Embryonic Age 1 week .

  • During the third week, if fertilization occurs , the fertilized egg (called a zygote) will begin producing the hormone human chorionic gonadotropin (the pregnancy hormone)
  • Human chorionic gonadotropin first becomes detectable in the mother's blood and urine between 6 and 14 days after fertilization (3 to 4 weeks gestational age).
  • During the 3rd week the sex of the fetus is determined by the father's sperm, and twins may be formed.
  • Fatigue and swollen or tender breasts are sometimes the first signs of pregnancy.

Gestational Age 4 weeks  ( 0.9 months)  . Embryonic Age 2 weeks

The embryo is the size of a pinhead.
Most pregnancy tests will be positive at this time .

Gestational Age 5  weeks  (1.2 months) . Embryonic Age 3 weeks
  • The brain, spine, and heart have begun to form. By the end of the week the heart will be pumping blood.
  • Week 5 is the beginning of the embryonic period which lasts from the the 5th to the 10th week.
  • It is during this critical period that many birth defects occur in the developing embryo.
  • Most of these birth defects will have no known cause or be due to a combination of factors (multifactorial).

 

Gestational Age 6  weeks  (1.4 months) . Embryonic Age 4 weeks

  • The embryo is now about the size of a pea.
  • The average crown to rump length is  about 0.2 inches ( 0.5 cm)
  • The eyes, nostrils , and arms are taking shape.
  • The heart is beating at about 110 beats per minute and sometimes may be seen using a transvaginal ultrasound at this time.

Gestational Age 7 weeks  (1.6 months) . Embryonic Age 5 weeks

  • The embryo is now about 0. 37 inches (0.95cm ) long
  • This week the hands and feet are forming as well as the mouth and face.
  • The heart is beating at about 120 beats per minute. Movement of the embryo can be detected by ultrasound.
  • By week 7 the trachea and bronchi of the lungs have formed , and the pseudoglandular stage of lung development begins [17]
  • Crown–rump length of 7 mm or greater and no heartbeat, or
    mean sac diameter of 25 mm or greater and no embryo is considered consistent with early pregnancy loss [20]

Gestational Age 8  weeks (1.8 months) . Embryonic Age 6 weeks
  The average embryo at 8 weeks  is 0.6 inches (1.6 cm)  long
  • The embryo is about the size of a bean. The fingers and toes are developing.
  • In a process is called physiological gut herniation , the intestine elongates and moves outside of the abdomen herniating into the base of the umbilical cord and rotate counter-clockwise at about 8 weeks . The intestine returns into the fetal abdomen by about 12 weeks [18] .

Gestational Age 9  weeks (2.1 months) . Embryonic Age 7 weeks

  • The heart is beating at about 170 beats per minute.
  • The average embryo at 9 weeks is 0.9 inches (2.3 cm)  long

Gestational Age 10  weeks (2.3 months)  . Fetal Age 8 weeks
 
  • The embryo's tail has disappeared and  it is now called a fetus. Fingerprints are being formed [11], and bone cells are replacing cartilage.
  • The average fetus at 10 weeks is 1.22 inches (3.1 cm)  long and weighs 1.2 ounces (35 grams) .

Gestational Age 11  weeks (2.5 months)  . Fetal Age 9 weeks
  • The fetus is starting to have breathing movements . It can open its mouth and swallow.
  • The average fetus at 11 weeks  is 1.6 inches (4.1 cm)  long and weighs 1.6 ounces (45 grams) .

Gestational Age 12 weeks  (2.8 months) . Fetal Age 10 weeks
 
  • The fetus is starting to make random movements .
  • The fetus begins to concentrate iodine in its thyroid and produce thyroid hormone at about this time.
  • The pancreas is beginning to make insulin, and the kidneys are producing urine.  The heart beat can usually be heard with and electronic monitor at this time.
  • The average fetus at 12 weeks is 2.1 inches (5.4 cm)  long and weighs 2.1  ounces (58 grams) .

Gestational Age 13 weeks  (3 months)  . Fetal Age 12 weeks
  The average fetus at 13 weeks is 2.6 inches (6.7 cm)  long and weighs 2.6 ounces (73 grams) .
  • All major organs are formed now, but they are too immature for the fetus to survive out of the womb.
  • Physiological gut herniation should be complete by this time
  • The fetal bladder can be consistently seen using ultrasound after 13 weeks [24].

Second Trimester of Pregnancy
(14 weeks and 0 days through 27 weeks and 6 days)

Gestational Age 14 weeks  (3.2 months) . Fetal Age 12 weeks
 
  • The  fetus's toenails are appearing . The gender may sometimes be seen
  • The average fetus at 14 weeks is  5.8 inches (14.7 cm)  long (crown t to heel) and weighs 3.3 ounces (93 grams) .

Gestational Age 15 weeks  (3.5 months) . Fetal Age 13 weeks
 
  • Fetal movement may be sensed now (called quickening). Some mothers don't feel the fetus moving until about 25 weeks.
  • The average fetus at 15 weeks is 6.6 inches (16.7 cm)  long and weighs 4.1 ounces (117 grams) .

Gestational Age 16 to 17 weeks  (3.7 to 3.9 months) . Fetal Age 14 to 15 weeks
  The average 16 week fetus is  7.3 inches (18.6cm)  long and weighs  5.2 ounces (146 grams) .
  • Hearing is beginning to form [12].
  • The canalicular period of lung development has started and will continue until  25 weeks [17]
The average 17 week fetus is 8 inches (20.4 cm)  long and weighs 6.4 ounces (181 grams) .
  • The pseudoglandular stage of lung development ends at about 17 weeks . There are still NO ALVEOLI (the air sacs in the lungs where the exchange of oxygen and carbon dioxide occurs), so respiration is not possible at this time [17].

Gestational Age 18 weeks (4.1 months) . Fetal Age 16 weeks
 
  • The ears are standing out, and the fetus is beginning to respond to sound.
  • The average 18 week  fetus is 8.7 inches (22.2 cm)  long and weighs 7.9 ounces (223 grams) .
  • The cerebellar vermis can be demonstrated to be fully formed on ultrasound  [22].

Gestational Age 19 weeks  (4.4 months)  . Fetal Age 17 weeks
  • The ears, nose and lips are now recognizable.
  • The average fetus at 19 weeks  is 9.5 inches (24 cm)  long and weighs 9.6 ounces (273 grams) .

Gestational Age 20 weeks  (4.6 months) . Fetal Age 18 weeks
 
  • The fetus is covered in fine hair (called lanugo) , has some scalp hair, and is capable of producing IgG and IgM ( two types of antibodies)
  • The average fetus at 20 weeks  is 10.1  inches (25.7 cm)  long and weighs 11.7 ounces (331 grams) .

Gestational Age 21 weeks  (4.8 months) . Fetal Age 19 weeks
  • The fetus is now able to suck and grasp, and may have bouts of hiccups. Some women may begin feeling Braxton Hicks contractions at this time.
  • The average fetus at 21 weeks  is 10.8 inches (27.4 cm)  long and weighs 14.1 ounces (399 grams) .
Gestational Age 22 weeks  (5.1 months) . Fetal Age  20 weeks
 
  • The average fetus at 22  weeks  is 11.4  inches (29 cm)  long and weighs 1.1 pound (478 grams) .
  • Survival and morbidity

Gestational Age 23 weeks  (5.3 months) . Fetal Age 21 weeks
 
  • The fetus is having rapid eye movements during sleep.
  • The average fetus at 23  weeks  is 12.1   inches (30.6 cm)  long and weighs 1.3 pounds  (568 grams) .
  • The entire corpus callosum may not be be seen using transabdominal  ultrasound before this age [23].
  • Survival and morbidity

Gestational Age 24  weeks (5.5 months) . Fetal Age 22 weeks
 
  • The average fetus at 24  weeks  is 12.7  inches (32.2 cm)  long and weighs 1.5 pounds  (670 grams) .
  • The terminal saccular stage of lung development  has started [17]
  • Survival and morbidity
Gestational Age 25 weeks  (5.8 months) . Fetal Age  23 weeks
 
  • The average fetus at 25  weeks  is 13.3  inches (33.7 cm)  long and weighs 1.7 pounds  (785 grams) .
  • The canalicular period of lung development is ending. Respiration is possible towards the end of this period [17,19]
    •
  • Survival and morbidity

Gestational Age 26  weeks  (6 months)  . Fetal Age 24  weeks
 
  • The fetus can respond to sounds that occur in the mother's surroundings. It's eyelids can open and close
  • The average fetus at 26  weeks  is 13.8  inches (35.1 cm)  long and weighs 2  pounds  (913 grams) .
  • Survival out of the womb at this age would be expected to be ~87[21]
Gestational Age 27 weeks  (6.2 months)  . Fetal Age 25 weeks
 
  • The average fetus at 27  weeks  is 14.4  inches (36.6 cm)  long and weighs 2.3  pounds  (1055 grams) .
  • Survival out of the womb at this age would be expected to be ~94%.[21]

Third Trimester of Pregnancy
(28 weeks 0 days through delivery)

Gestational Age 28  weeks  (6.4  months) . Fetal Age 26  weeks
  The fetus has eyelashes and it's skin is red and covered with vernix caseosa a waxy substance that is believed to act as a protective film with anti-infective and waterproofing properties.
  • The average fetus at 28  weeks  is 14.9  inches (37.9 cm)  long and weighs 2.7 pounds  (1210 grams) .
  • Survival out of the womb at this age would be expected to be ~94% [21]
Gestational Age 29  to 31 weeks  (6.6 months to 7.1 months) . Fetal Age 27 to 29 weeks
 
  • The average fetus at 29  weeks  is 15.4  inches (39.2 cm)  long and weighs 3 pounds  (1379 grams) .
  • The average fetus at 30  weeks  is 16  inches (40.5 cm)  long and weighs 3.4  pounds  (1559 grams) .
  • The average fetus at 31  weeks  is 16.5  inches (41.8 cm)  long and weighs 3.9 pounds  (1751  grams) .
    •

Gestational Age 32 to 33 weeks  (7.4 to 7.6 months) . Fetal Age 30 to 31 weeks
The  fetus is forming muscle and storing body fat.

 If the fetus is a boy, his testicles are descending.

  • The average fetus at 32  weeks  is 16.9  inches (43 cm)  long and weighs 4.3 pounds  (1953 grams) .
  • The average fetus at 33  weeks  is 17.3  inches (44 cm)  long and weighs 4.8  pounds  (2162 grams) .
    • T he distal femoral epiphysis ossification center can usually  be seen in 72 % of fetuses at 33 weeks [13,14]

Gestational Age 34 to 36 weeks  (7.8  to 8.3 months) . Fetal Age 32 to 34 weeks
  The fetus is now considered to be late preterm
  • The average 34  week fetus  is  17.8  inches (45.2 cm)  long and weighs 5.2 pounds  (2377 grams)
  • The average 35 week fetus  is  18.2  inches (46.3 cm)  long and weighs 5.7 pounds  (2595 grams)
    • The proximal tibial epiphysis ossification center may be seen in 35 % of fetuses at 35 weeks [13,14]
  • The average 36 week fetus  is  18.6  inches (47.3 cm)  long and weighs 6.2 pounds  (2813 grams)

Gestational Age 37 to 38 weeks ( 8.5  to 8.7 months) . Fetal Age 35 to 36 weeks
  The fetus is now considered to be early term
  • The average 37 week fetus  is  19.   inches ( 48.3 cm)  long and weighs 6.7  pounds  (3028 grams)
  • The average 38 week fetus  is  19.4  inches (49.2 cm)  long and weighs 7.1 pounds  (3236 grams)
    •  The proximal humeral epiphysis ossification center may be seen at 38 weeks [15]

Gestational Age 39 to 41 weeks  (9  to 9.4 months) . Fetal Age 37 to 39 weeks
The fetus is now full term.
  • The average 39  week fetus  is  19.7  inches (50.1 cm)  long and weighs 7.6 pounds  (3435 grams)
  • The average 40 week fetus  is  20.1  inches (51 cm)  long and weighs 8 pounds  (3619 grams)
  • The average 41 week fetus  is  20.4  inches (51.8 cm)  long and weighs  8.3  pounds  (3787 grams) .

Equations

Crown to Rump Length
LN (MA) =1.684969 + (.315646 CRL) - (.049306CRL^2) + (.004057 CRL^3) - (.000120456 CRL^4)
Hadlock FP et. al.,Fetal crown-rump length: reevaluation of relation to menstrual age (5-18 weeks) with high-resolution real-time US. Radiology. 1992 Feb;182(2):501-5.PMID: 1732970<br>
Crown to Heel
Length (cm)= -0.0219^2 * Gestational age (weeks) + 2.5764* Gestational age (weeks) -17.059

Equation extrapolated from FIGURE 2 in Fenton TR. A new growth chart for preterm babies: Babson and Benda's chart updated with recent data and a new format. BMC Pediatr. 2003 Dec 16;3:13. doi: 10.1186/1471-2431-3-13. PMID: 14678563; PMCID: PMC324406.
Weight
ln weight (g)=0.578 + 0.332*MA -.00354*MA^2
Hadlock FP, et al., In utero analysis of fetal growth: a sonographic weight standard.Radiology. 1991 Oct;181(1):129-33.PMID: 1887021

Reviewed 1/2/2023

References

 

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