All mammals, including humans, go through a period of gestation that lasts from conception to birth. During this time, your body goes through many changes and may be more susceptible to developing diabetes or hypertension.

When you’re pregnant, you might hear the word “gestation” often. Here, we’ll explore specifically how gestation relates to human pregnancy.

We’ll also discuss some of the similar terms you might encounter throughout your pregnancy — such as gestational age and gestational diabetes.

What is gestation?

Gestation is defined as the time between conception and birth. Though we’re focusing on human gestation, this term applies more broadly to all mammals. A fetus grows and develops in the womb during gestation.

Gestation period

The gestation period is how long a woman is pregnant. 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.

Gestational age

The actual date of conception generally isn’t known for humans, so gestational age is the common way to measure how far along a pregnancy is. Where your baby is in their development — such as whether their fingers and toes have formed — is tied to gestational age.

Gestational age is measured in weeks from the first day of your last menstrual period. 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 sperm fertilizes an egg.

Your doctor can also determine gestational age using an ultrasound or after delivery.

During an ultrasound, your doctor will measure your baby’s head and your abdomen to determine gestational age.

After birth, gestational age is determined using the Ballard Scale, which assesses the physical maturity of your baby.

Gestational age is divided into two periods: embryonic and fetal. The embryonic period is week 5 of pregnancy — which is when the embryo implants in your uterus — to week 10. The fetal period is week 10 to birth.

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.

How to calculate a due date

The most accurate way to find out your due date is for your doctor to calculate it using an ultrasound in the first trimester. Your doctor will use certain measurements to figure out how far along you are already.

You can also estimate your due date using the following method:

  1. Mark down the day your last period started.
  2. Add seven days.
  3. Count back three months.
  4. Add a year.

The day you end up on is your due date. This method assumes that you have a regular menstrual cycle. So while it’s not perfect, it’s a good estimate in most cases.

Gestational diabetes

Gestational diabetes is a type of diabetes that a woman can develop during pregnancy. It usually develops after week 20 of pregnancy and goes away after delivery.

Gestational diabetes happens because the placenta produces hormones that keep insulin from working correctly. This raises your blood sugar and causes diabetes.

Doctors aren’t sure why some women get gestational diabetes and some don’t. However, there are certain risk factors, including:

  • being older than 25
  • having type 2 diabetes or having a family member with type 2 diabetes
  • having gestational diabetes in a previous pregnancy
  • previously giving birth to a baby over 9 pounds
  • being overweight
  • having black, Hispanic, Native American, or Asian heritage

Many women with gestational diabetes don’t have any symptoms. Your doctor will assess your risk when you first get pregnant, and then continue to test your blood sugar throughout pregnancy.

Gestational diabetes can often be controlled with a healthy lifestyle, including regular exercise (if your doctor says it’s OK) and a nutritious diet that includes lots of leafy vegetables, whole grains, and lean proteins. A healthy lifestyle can also help reduce your risk of gestational diabetes.

Some women might also need medication to help control gestational diabetes.

Keeping your blood sugar in control is very important. If not controlled, gestational diabetes can cause problems for both you and your baby, including:

  • preterm birth
  • respiratory issues for your baby
  • being more likely to need a cesarean delivery (commonly known as a C-section)
  • having very low blood sugar after delivery

Gestational diabetes also increases your risk for type 2 diabetes. If you have gestational diabetes, you should have your blood sugar checked regularly after delivery.

Gestational hypertension

Gestational hypertension is a type of high blood pressure that can develop during pregnancy. It’s also called pregnancy-induced hypertension (PIH).

PIH develops after week 20 and goes away after delivery. It’s different from preeclampsia, which also involves high blood pressure but is a more serious condition.

Hypertension affects approximately 6 to 8 percent of those who are pregnant. Women at increased risk of PIH include those who:

  • are pregnant for the first time
  • have close family members who’ve had PIH
  • are carrying multiples
  • previously have had high blood pressure
  • are under 20 or over 40

Many women with PIH don’t have symptoms. Your provider should check your blood pressure at every visit, so they know if it starts increasing.

Treatment depends on how close to your due date you are and how severe the hypertension is.

If you’re close to your due date and your baby is developed enough, your doctor might have you deliver. If your baby isn’t yet ready to be born and your PIH is mild, your doctor will monitor you until the baby is ready to be delivered.

You can help reduce your blood pressure by resting, eating less salt, drinking more water, and lying on your left side, which takes your weight off major blood vessels.

In addition, if your baby isn’t developed enough to be born but your PIH is more severe, your doctor might recommend blood pressure medication.

PIH can lead to a low birth weight, but most women with the condition deliver healthy babies if it’s caught and treated early. Severe, untreated PIH can lead to preeclampsia, which can be very dangerous for both mom and baby.

There’s no sure way to prevent PIH, but there are some ways to reduce your risk, including:

  • eating a healthy diet
  • drinking lots of water
  • limiting your salt intake
  • elevating your feet a few times a day
  • exercising regularly (if your doctor says it’s OK)
  • making sure you get enough rest
  • avoiding alcohol and caffeine
  • making sure your provider checks your blood pressure at every visit

The bottom line

“Gestation” refers to the amount of time you’re pregnant. It’s also used as part of many other terms related to different aspects of pregnancy.

Gestational age helps your doctor figure out if your baby is developing as it should. Find out more about how your baby develops during gestation.