What is gestation?
Gestation is defined as the time between conception and birth. This applies to all viviparous animals such as mammals, but we will be focusing specifically on human development. A fetus grows in the pregnant parent’s uterus during gestation.
The gestation period describes the duration of pregnancy. Most babies are born between 38 and 42 weeks of gestation. Babies born before 37 weeks are considered premature. Babies born after 42 weeks are called postmature.
Why is it important?
Gestational age helps to estimate a possible due date, plan antenatal care and testing, and evaluate the baby’s health at birth. It provides a guideline to determine if the baby is growing as expected and when to perform prenatal screening. Babies born smaller or bigger than expected for their gestational age may need to be monitored more closely.
How is gestation calculated?
The actual date of conception isn’t known for certain in humans, so gestational age is the best way to measure how far along a pregnancy is. Gestation age is related to each specific stage of development, and all babies grow at roughly the same rate at the earliest stage of pregnancy.
Gestational age is measured in weeks from the first day of your last menstrual period (LMP). This means that your last period counts as part of your pregnancy. Even though you weren’t actually pregnant, your period is a signal that your body is preparing for pregnancy.
Fetal growth doesn’t actually start until conception, which is when a single sperm fertilizes an egg.
The estimated due date (EDD) is the date that spontaneous onset of labour is expected to occur. The EDD may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP). This is the method used by pregnancy calculators. The accuracy of the EDD derived by this method depends on the mother’s precise recollection of dates, assumes regular 28 day cycles, and that ovulation and conception occurs on day 14 of the cycle. Establishing the EDD by LMP may under or overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks.
In cases where the date of conception is known precisely, such as with in vitro fertilization (IVF), the EDD is calculated by adding 266 days to the date of conception.
Ultrasound is used to determine the size of the fetus and therefore the gestational age (the time elapsed since the first day of the last menstrual period). The accuracy of the ultrasound estimate of the gestational age varies according to the timing of the scan (and therefore the gestation itself), the resolution of the ultrasound image, and the position of the fetus.
Gestational age is divided into two distinct periods: embryonic and fetal.
The embryonic period extends from the beginning of the third week to the end of the eighth week of intrauterine life. It is also called the period of organogenesis.
The fetal period extends from the beginning of the ninth week (third month) until the birth. During this period, there is exceptional growth and specialization of the body structures, characterised by the maturation of tissues and organs.
Gestational age is most accurately determined in the First Trimester (the first three months of pregnancy) by measuring the Crown Rump Length (CRL) of your baby on an ultrasound scan; this measurement from the top of the fetal head to the rump is normally performed at 12 weeks from the LMP.
If you attend your first scan after the first trimester, the most accurate measurement to assess the gestation of the pregnancy is the Head Circumference (HC). This is performed from 14weeks and 1day.
In later stages of pregnancy, the sonographer will measure specific parts of the baby’s body including the Abdominal Circumference (AC) , Head Circumference (HC), and Femur (thigh bone) Length (FL) to confirm gestational age and fetal growth. The gestational age is never re-dated at this scan.
Gestational age vs fetal age
While gestational age is measured from the first day of your last menstrual period, fetal age is calculated from the date of conception. This is during ovulation, which means that fetal age is about two weeks behind gestational age.
This is the actual age of the fetus. However, it’s a less precise way to measure pregnancy, because in most cases it’s impossible to know when conception actually happens in humans.
Other methods to determine gestational age
- Human Chorionic Gonadotropin Pregnancy Test
Human chorionic gonadotropin (HCG) first becomes detectable in the mother’s urine between 6 and 14 days after fertilization (3 to 4 weeks gestational age). The gestational age would, therefore, be at least 3 to 4 weeks at the time of a reliable hCG pregnancy test.
Some digital pregnancy tests show the gestational age, but this is only an assessment of the hormone level and may be inaccurate in a twin pregnancy, or if there are specific conditions which cause the hormone levels to rise abnormally.
- Pelvic Examination
A pelvic examination by an experienced midwife or doctor, supported by good menstrual records in the first trimester has been shown to be a reliable method for dating of pregnancy. However, this has been superseded by the use of ultrasound, which is much more accurate.
The Estimated Due Date
While the EDD can be calculated using ultrasound scans, it is still an estimation and the actual length of a pregnancy depends on various factors including maternal age, length of previous pregnancies, and weight of the mother at birth.
However, there are still more factors affecting natural variation in the length of gestation that are not well understood. Studies have shown that fewer than 4% of births occur on the exact due date, 60% occur within a week of the due date, and almost 90% occur within two weeks of the due date. Therefore, while it is possible to be fairly confident that a baby will be born within about two weeks of the due date, it is currently not possible to predict the exact day of birth with certainty.
Gestational Age at Birth
At birth, a baby’s condition is evaluated by a doctor to determine how the baby relates to their gestational age. Their height, muscle tone, head circumference, hair and skin condition, reflexes, weight, vital signs, muscle tone and posture are assessed.
Based on those factors and how they compare to their calendar age, the baby is considered small for gestational age (SGA), large for gestational age (LGA), or appropriate for gestational age (AGA). In terms of weight, full-term infants who are AGA weigh on average between 5.5 lbs / 2.5 kg and 8.75 lbs / 4kg.
Babies who are small or large may be monitored more closely for complications.