Is the Day of Birth Day 1? How Newborn Age Is Counted
Your baby has arrived. Friends ask how old the little one is, and suddenly a simple question gets complicated: is today day one, or is day one tomorrow? Doctors, growth charts, family traditions and parenting apps can all count differently. This guide explains the common counting systems with clear examples, so you can work out your baby’s age correctly in any situation.
There is no single answer, because it depends on who is counting. Modern medicine and most countries count the birth day as day 0: a baby is 0 days old on the day of birth, and turns 1 day old on the next day.
Traditional celebrations often count the birth day as day 1 instead. The Chinese 100-day celebration, the Japanese o-kuizome and the Korean baek-il all count from the birth day as the first day. Traditional age systems in East Asia even make a newborn 1 year old at birth. Knowing which system a situation uses is the whole trick.
Why parents get confused
The confusion is normal, because different contexts use different conventions. Your baby’s vaccination record and the doctor use the medical day count. The growth charts from WHO and CDC plot age in completed days. Meanwhile grandparents may talk about a full month, a hundred-day celebration or a traditional age that starts at one.
- Hospital notes and checkups: medical counting, birth day is day 0
- Vaccination schedules: stated in weeks and months from birth
- Growth charts (WHO, CDC): exact age in completed days
- Family celebrations: traditional counting, birth day is day 1
- Traditional age (East Asia): a newborn can be 1 year old at birth
The medical standard: the birth day is day 0
Medical records, vaccination schedules and the WHO and CDC growth standards all count age as completed days: the number of full days since birth. A baby born at 9 a.m. on the 1st is 0 days old that day, 1 day old on the 2nd, and 30 days old on the 31st.
The neonatal period is defined as the first 28 days of life, from day 0 to day 27. Weeks and months follow the same pattern: a baby is 0 months old until the same date one month later, when they turn 1 month old. Age in months follows the calendar date, not fixed 30-day blocks.
Why does medicine count this way? A date-based calculation is exact, easy to verify and identical everywhere in the world, which matters for vaccines, medication doses and growth tracking.
- 1st of the month, 9 a.m.: born. Age: 0 days
- 2nd of the month: 1 day old
- Same month, 31st: 30 days old
- 1st of next month: 1 month old (31 days)
Traditional and local ways of counting
In Western countries a baby is 0 years old at birth and turns 1 on the first anniversary of the birth date. This is also the international standard for legal and administrative age.
Traditional East Asian age systems work differently. In the Chinese system of virtual age a child is 1 at birth and gains a year at each Lunar New Year. Korea used a similar system in which everyone aged on January 1, until the law unified official counting to international age on June 28, 2023. In Vietnam, traditional counting also starts at 1.
For celebrations, the convention often flips to counting the birth day as day 1. The 100-day celebration in China, Japan and Korea counts from the birth day, so it falls about 99 full days after birth. The Chinese full-month celebration falls on the same date one month after birth.
One legal curiosity: some common-law jurisdictions, such as parts of the United States, follow the old rule that a person reaches a legal age on the day before their birthday.
How to calculate your baby’s age
For days: subtract the birth date from today. The result is the age in completed days, with the birth day counting as day 0. Any calendar app can do this, and most parenting apps show it automatically.
For months: a baby turns 1 month old on the same date of the next month. If that date does not exist, use the last day of that month. A baby born on January 31 is 1 month old on February 28 (or 29 in a leap year).
For babies born early: use corrected age. Doctors count from the due date instead of the birth date until about two years of age, so growth and milestones are compared fairly.
Practical tip: pick one convention and stay consistent, especially for medical visits. Write down the date of birth and the date of each checkup, and let the numbers do the talking.
Sources
This article is based on the following public references. Regional customs vary, so local sources may describe slightly different practices.
- WHO Child Growth Standards
- WHO Growth Charts (CDC, recommended for use in the United States)
- Gestational Age (MSD Manual Professional Edition)
- How the Day of Birth Affects Benefits (US Social Security Administration)
- Age data in population censuses (UN Statistics Division)
This article is for general information and is not medical advice. Feeding, development and health questions should always be discussed with a pediatrician or another qualified professional. The traditions described here come from public references and can vary by family and region.
Track every day from day zero
Every day counts. Keep them all.
Log feeding, sleep, diapers and growth in one tap. Your baby’s age in days, weeks, months and percentiles is calculated automatically, and the whole family stays in sync.
Common questions
Is the day of birth day 0 or day 1?
In medicine and most everyday use, the birth day is day 0 and your baby turns 1 day old on the next day. Traditional ceremonies such as the 100-day celebration count the birth day as day 1, which is why the same day can be described two ways.
When is a baby 1 month old?
On the same date of the next month. A baby born on March 5 is 1 month old on April 5, 2 months old on May 5, and so on. Age in months follows the calendar, not 30-day blocks.
How is the 100-day celebration counted?
Traditionally the birth day is counted as day 1, so the 100th day falls 99 full days after birth, about 3 months and 10 days. In medical day-0 counting, the same moment is described as 99 days old.
Why do some cultures say a baby is 1 year old at birth?
Traditional age systems in China, Korea and Vietnam count the time in the womb as the first year. Korea unified official counting to international age on June 28, 2023, but traditional counting still appears in daily life.
How is corrected age calculated for premature babies?
Corrected age counts from the expected due date instead of the birth date. For a baby born 8 weeks early, the corrected age is the actual age minus 8 weeks. Doctors use corrected age for growth and milestones until about 2 years of age.
Do vaccines and checkups use day-0 counting?
Yes. Vaccination schedules and growth charts are based on completed weeks, months and days from birth, with the birth day as day 0. When in doubt, give your doctor the exact dates and let them calculate.