Pregnancy Week Calculator

About Pregnancy Week Calculator

How Pregnancy Weeks Are Counted

Pregnancy is dated from the first day of the last menstrual period, not from conception. This is the standard method used by both ACOG (the American College of Obstetricians and Gynecologists) and the NHS. It means gestational age includes roughly two weeks before fertilisation actually occurred.

The formula is straightforward:

Gestational age = Today's date - LMP date

Due date = LMP + 280 days (40 weeks)

For example, if your LMP was 1 January 2026 and today is 12 April 2026, that is 101 days, which puts you at week 14 and day 3 of pregnancy - the start of the second trimester.

If you only know your due date (from an ultrasound or your doctor), the calculator works backwards: LMP = Due date - 280 days. The result gives you the same week and trimester information.

According to a 2013 study in BMC Pregnancy and Childbirth, the average duration of a singleton pregnancy is 268 days from ovulation (roughly 282 from LMP), though 280 days remains the clinical standard. Only about 4-5% of babies arrive on their exact due date. Most births fall within a two-week window either side.

It is worth understanding the difference between gestational age and fetal age. Gestational age counts from the LMP, so it is always about two weeks ahead of fetal age (which counts from conception). When your doctor says you are 12 weeks pregnant, the baby has actually been developing for about 10 weeks. This distinction matters when reading about fetal development, since some sources use one system and some use the other.

The 280-day standard is based on Naegele's rule, named after a German obstetrician who published the formula in 1812. Despite its age, it remains the most widely used calculation in clinical practice. Modern studies have confirmed it is reasonably accurate for women with regular 28-day cycles, though it tends to slightly underestimate the true average. A large Norwegian study found that first-time mothers delivered at a median of 283 days, while women who had given birth before delivered at a median of 280 days.

Baby Size Week by Week - Fruit Comparisons

One of the most popular ways to visualise baby growth is comparing it to everyday fruits and vegetables. These comparisons are based on average crown-to-rump length (CRL) during the first 20 weeks and crown-to-heel length after that, as published by the WHO and ACOG.

Week	Size	Length	Key Development
4	Poppy seed	1mm	Implantation, positive pregnancy test
6	Lentil	4mm	Heartbeat may be detectable
8	Raspberry	1.5cm	Fingers and toes forming, first prenatal visit
10	Strawberry	3cm	Bones and cartilage forming
12	Plum	5.5cm	End of first trimester, organs formed
14	Peach	8.5cm	Body growing faster than head
16	Avocado	11.5cm	Can hear sounds, eyes sensitive to light
18	Sweet potato	14cm	First movements may be felt
20	Banana	16.5cm	Halfway point, anatomy scan
24	Corn on the cob	30cm	Viability milestone reached
28	Aubergine	38cm	Third trimester begins, baby can blink
32	Squash	42cm	Practising breathing, nails fully formed
36	Romaine lettuce	47cm	Baby may drop into pelvis
40	Pumpkin	51cm	Full term, ready for delivery

The jump from about 16cm to 27cm between weeks 20 and 21 reflects the switch from crown-to-rump (CRL) to full crown-to-heel measurement. Growth is not always linear, and individual babies vary considerably. Your midwife or doctor will track your baby's specific growth via ultrasound measurements and plot them against standardised growth charts.

Weight gain follows a different pattern to length. In the first trimester, the embryo weighs less than 30 grams. By week 20, a typical fetus weighs around 300g. From there, weight gain accelerates rapidly. By week 28, average weight is about 1kg. By week 36, it is around 2.7kg. At full term (week 40), the average baby weighs 3.3-3.5kg, though anything from 2.5kg to 4.5kg is considered within the normal range according to the WHO.

Trimester Breakdown and What to Expect

Pregnancy is divided into three trimesters, each with distinct developmental milestones and common symptoms.

Trimester	Weeks	Baby Development	Common Symptoms
First	1-12	All major organs form, heartbeat from week 6, limbs develop	Nausea, fatigue, breast tenderness, frequent urination
Second	13-27	Rapid growth, first movements felt (weeks 18-22), anatomy scan at week 20, viability from week 24	Nausea often subsides, more energy, back pain may start
Third	28-40	Lungs mature, brain develops rapidly, baby gains most of its weight	Fatigue returns, Braxton Hicks contractions, shortness of breath, swelling

The second trimester is often called the "golden period" because many of the first-trimester symptoms like nausea fade while the baby is still small enough to be comfortable. By the third trimester, the baby is gaining roughly 200-250g per week as it prepares for life outside the womb.

Morning sickness affects about 70-80% of pregnant women according to ACOG, most commonly between weeks 6 and 12. Despite the name, it can happen at any time of day. For most women, it resolves by week 14-16, though a small percentage (about 1-3%) experience a severe form called hyperemesis gravidarum that may require medical treatment.

The first movements (called quickening) are typically felt between weeks 18 and 22. First-time mothers tend to notice them closer to week 20-22, while women who have been pregnant before often recognise the sensation earlier, around week 16-18. Initially, movements feel like flutters or bubbles. By the third trimester, kicks and rolls become much more obvious.

If you are tracking recommended weight gain during pregnancy, the IOM guidelines suggest most weight is gained during the second and third trimesters - about 0.4-0.5kg per week for women with a normal pre-pregnancy BMI. Total recommended gain for a normal-weight woman (BMI 18.5-24.9) is 11.5-16kg over the full pregnancy.

Key Pregnancy Milestones by Week

Week	Milestone
4	Implantation complete - pregnancy test may show positive
6	Heartbeat detectable on ultrasound
8-12	First prenatal visit and dating scan
12	End of first trimester - miscarriage risk drops to under 2% (ACOG)
18-22	Anatomy scan (mid-pregnancy ultrasound)
24	Viability threshold - survival possible with neonatal intensive care
28	Third trimester begins
37	Early term - lungs typically mature enough for delivery
39-40	Full term - optimal timing for birth
41-42	Late/post-term - induction typically discussed

According to the NHS, the 12-week scan is one of the most important early appointments. It confirms the due date (often adjusting it based on the baby's CRL measurement) and screens for certain conditions such as Down's syndrome. The 20-week anatomy scan checks the baby's organs, bones, and growth in detail. It is also when many parents learn the sex of the baby, though this is not always offered depending on the hospital or clinic.

The viability milestone at week 24 is significant. Before this point, a baby born prematurely has very limited chances of survival even with intensive care. At 24 weeks, survival rates with neonatal intensive care are roughly 40-70%, depending on the hospital and specific circumstances. By 28 weeks, survival rates increase to about 80-90%, and by 32 weeks they exceed 95%. These figures come from large-scale data published by the National Institute for Health and Care Excellence (NICE) and the Vermont Oxford Network.

Term Classifications and What They Mean

Not all "full term" pregnancies are the same. ACOG refined the definition of term pregnancy in 2013 to give more specific guidance:

Classification	Weeks	What It Means
Early term	37-38	Baby is likely healthy but benefits from a few more weeks of brain and lung development
Full term	39-40	Optimal timing for birth - organs fully mature
Late term	41	Still normal, but increased monitoring recommended
Post-term	42+	Induction typically offered due to slightly increased risks

This distinction matters because research shows babies born at 39-40 weeks have fewer breathing problems, better temperature regulation, and stronger feeding ability compared to those born at 37-38 weeks. The difference is small for any individual baby, but across large populations it is statistically significant. For this reason, ACOG and NICE both recommend against elective delivery before 39 weeks unless there is a medical reason.

When Should Key Prenatal Appointments Happen?

Knowing your current week helps you plan ahead for standard prenatal check-ups. The schedule below follows ACOG and NHS guidelines for a low-risk singleton pregnancy. Your provider may adjust the timing based on your individual circumstances.

Week	Appointment / Test	Purpose
8-10	First prenatal visit (booking appointment)	Medical history, blood tests (blood type, Rh factor, iron, infections), urine tests, BMI check
11-14	Dating scan and nuchal translucency screening	Confirms due date via CRL measurement, screens for chromosomal conditions
15-20	Quad screen or NIPT results review	Blood test results for Down's, Edwards', and Patau's syndromes
18-22	Anatomy scan (anomaly scan)	Detailed check of baby's organs, spine, limbs, placenta position. Sex can often be determined.
24-28	Glucose tolerance test	Screens for gestational diabetes. Also Rh antibody check if Rh-negative.
28-36	Fortnightly midwife visits	Blood pressure, fundal height, baby's position, urine protein check
36	Group B strep swab (US) / birth plan discussion	GBS screening (offered routinely in the US, risk-based in the UK). Discuss birth preferences.
36-40	Weekly visits	Monitoring for pre-eclampsia, baby's position, cervical checks if indicated
41+	Post-dates monitoring	Non-stress test and amniotic fluid check. Induction typically offered between 41 and 42 weeks.

First-time mothers in the UK typically have around 10 appointments throughout pregnancy, while those who have given birth before have around 7. In the US, the traditional schedule is monthly visits until week 28, fortnightly until week 36, then weekly until delivery - though many providers now use fewer visits for low-risk pregnancies after research by the WHO showed similar outcomes with reduced schedules.

When to Use This Calculator vs a Due Date Calculator

The Pregnancy Due Date Calculator focuses on estimating when your baby will arrive. This pregnancy week calculator is designed for ongoing use throughout pregnancy - it shows your current position in the journey, what is happening with the baby this week, and how far along you are.

If you are planning a pregnancy or tracking your fertile window, the Ovulation Calculator can help you identify your most fertile days based on your cycle length. The average menstrual cycle is 28 days, with ovulation typically occurring around day 14, but cycles between 21 and 35 days are considered normal by ACOG.

All calculations run entirely in your browser. No personal health data is stored or sent to any server.

Sources

• ACOG - How Your Fetus Grows During Pregnancy
• NHS - Pregnancy Due Date Calculator and Week-by-Week Guide
• WHO - Child Growth Standards
• ACOG Committee Opinion - Definition of Term Pregnancy
• NICE Guideline NG201 - Antenatal Care
• BMC Pregnancy and Childbirth - Gestational Duration Research