Pregnancy weeks, due dates and notations like 18+3 can be confusing, especially when the due date may still change after a scan. Based on Finnish health sources and others, this guide explains how pregnancy length is calculated, and based on the app’s source code, how Bloom Journey calculates the weeks.
Pregnancy length is counted from the first day of your last period and written in completed weeks and days, for example 18+3. The due date is the day when the pregnancy reaches 40+0 weeks, or 280 days. An early pregnancy scan measures the baby and can refine the due date. With fertility treatment, the due date is calculated from the embryo transfer date. Only a few percent of births happen on the due date itself.
Pregnancy length is counted from the first day of your last period, even though conception happens later. In a typical 28-day cycle, the first two weeks or so of pregnancy are therefore the time before conception.
The length is written in completed weeks and days. For example, 18+3 means the pregnancy has lasted 18 full weeks and 3 days. In Finland, pregnancy weeks are abbreviated rv or rvk, and the Finnish version of Bloom Journey shows, for example, rv 18+3.
So week 18 means the period 18+0 to 18+6. Some people describe the same days as the 19th week, because 18 full weeks have already passed. Bloom Journey shows completed weeks.
The due date is the day when 40+0 weeks, or 280 days, have passed since the first day of your last period. According to the Finnish health library Terveyskirjasto, a 28-day cycle is assumed at the first antenatal visit, but the cycle length should be taken into account if it differs clearly from 28 days.
The due date is an estimate. According to Terveyskirjasto, only a few percent give birth on the due date itself, and the vast majority, over 90 percent, give birth between weeks 38 and 42.
Bloom Journey shows your due date and counts the days to it. Once the due date has passed, the app tells you how many days have gone by and reminds you that your care team will guide the next steps.
The date of your last period is not always known or exact. Then the pregnancy is dated in another way.
In Finland, pregnant women are offered an early pregnancy ultrasound scan between weeks 10+0 and 13+6. One of its aims is to confirm the length of the pregnancy based on the baby’s size, so the due date may be adjusted by a few days.
When a pregnancy began with an embryo transfer, the due date is calculated from the transfer date and the embryo’s age. According to the American College of Obstetricians and Gynecologists (ACOG), the due date is 261 days after the transfer for a day-5 embryo and 263 days for a day-3 embryo.
If the conception date is known, Bloom Journey sets the due date 266 days later, because it treats conception as day 14 of pregnancy. The app uses the insemination date as the conception date.
If you do not know a date yet, you can start Bloom Journey with an estimate of your pregnancy week. When you get a due date from your clinic or a scan, add it to the app. According to the app, a due date from your clinic or a scan is the most accurate.
Pregnancy is divided into three trimesters, but the boundaries vary between sources. In Bloom Journey the first trimester covers weeks 0+0 to 13+6, the second starts at 14+0 and the third at 28+0. After the birth, the app continues for 12 weeks with the so-called fourth trimester.
According to Terveyskirjasto, a pregnancy is post-term when it has lasted more than 42+0 weeks. Bloom Journey’s daily notes use ACOG’s categories: early term 37+0 to 38+6, full term 39+0 to 40+6, late term 41+0 to 41+6 and post-term from 42+0. The app follows a pregnancy up to 42+6.
Bloom Journey calculates the weeks and the due date on your phone. The method depends on what you tell it.
From the first day of your last period. That is why some pregnancy weeks have already passed before the pregnancy has actually begun.
The due date is 280 days, or 40+0 weeks, from the first day of your last period. If your cycle differs clearly from 28 days, its length should be taken into account. An early pregnancy scan can refine the due date.
The pregnancy has lasted 18 full weeks and 3 days. So the period 18+0 to 18+6 is pregnancy week 18, although some people describe the same days as the 19th week.
According to Terveyskirjasto, only a few percent. The vast majority, over 90 percent, are born between weeks 38 and 42.
An early pregnancy scan measures the baby, and the length of the pregnancy can be estimated more precisely from that than from the date of your period. That is why the due date may move by a few days.
From the transfer date and the embryo’s age. According to ACOG, the due date is 261 days after the transfer for a day-5 embryo and 263 days for a day-3 embryo. Bloom Journey calculates it the same way and uses 260 days for a day-6 embryo.
According to Terveyskirjasto, a pregnancy is post-term when it has lasted more than 42+0 weeks.
This guide gives general information on calculating pregnancy length and does not replace advice from your maternity clinic or doctor. Bloom Journey is available for Android phones, and an iPhone version is on the way. The app is in English, Finnish, Spanish, German and Russian.
The information on calculating pregnancy length is based on these sources, checked on 6 November 2026. Bloom Journey’s method was checked against the app’s source code.
The guide explains how to set up the Bloom Journey pregnancy app and what you can do with it, from following your pregnancy week by week to memories and privacy.
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