8 common questions about pregnancy dating or “how to date a pregnancy?”
There is a lot of confusion about pregnancy dating and the various scenarios that are possible, for example with:
- A natural pregnancy,
- Assisted pregnancy,
- Pregnancy after a miscarriage
Let’s cover some of the most common scenarios:
- Pregnancy dating with regular cycles, for a naturally conceived pregnancy
- Pregnancy dating with irregular cycles, for a naturally conceived pregnancy
- Pregnancy dating with IVF and a fresh embryo transfer
- Pregnancy dating with IVF and a frozen embryo transfer (FET)
- Day 5, Day 6 or Day 7 embryo – does this change the dating?
- Pregnancy dating after a miscarriage
- Pregnancy dating with ultrasounds
- Pregnancy due date
Pregnancy dating with regular cycles, for a naturally conceived pregnancy
First, let’s talk about a naturally conceived pregnancy, and let’s assume your cycles are regular and about 28-30 days long. The first day of bleeding is called cycle day 1. Approximately two weeks later you ovulate. Then sex happens, followed by pregnancy! Your period is due about four weeks from the first day of your period. The period doesn’t come, you do a pregnancy test, and bam – you’re pregnant(!). At this point in the pregnancy, which is the first time that you did a home pregnancy test (which is the time your period was due), you’re already four weeks pregnant. Now, this is very counterintuitive because you might think that, four weeks ago I was on my period so clearly I wasn’t pregnant..so how can I be four weeks pregnant? The reason for this is: that it’s just convention! For the egg that led to that pregnancy, you can think of it as starting to grow and developing in the ovary and it started on day one of your menstrual cycle.
To summarize: if you have regular cycles (about 28-30 days) and you conceived naturally and you’re due for a period at a regular time and you do a pregnancy test and it’s positive – at that point you’re approximately four weeks pregnant!
In this same scenario, doing an ultrasound can confirm the dates. An ultrasound will measure what is called the crown rump length, which is the size of the fetus and that would be another estimation of your due date.
Pregnancy dating with irregular cycles, for a naturally conceived pregnancy
Now, let’s say you don’t have regular cycles. A common scenario is, “I haven’t had my period for three months. What’s going on?”
The first step is to do a home pregnancy test. If it’s positive, then we would do blood work to look at the pregnancy hormone level. Sometimes the level can give us a clue as to how far along you are. With irregular cycles, it’s hard to date the pregnancy based on menstrual cycles! The only way to date it accurately is based on an ultrasound. It’s not always possible to date a pregnancy based on a menstrual cycle when your cycles are long or irregular and that’s totally normal because pregnancy dating is really based on a 28-day cycle. And we know lots of women don’t have a 28-day cycle.
Pregnancy dating with IVF and a fresh embryo transfer
Now let’s talk about pregnancies that are conceived through IVF through a fresh embryo transfer. This means a woman will take injection medications, proceed with an egg retrieval, and have a fresh embryo transfer either 3 or 5 days later.
You take a pregnancy test and it’s positive(!). In this situation, you consider the day of retrieval is about equal to ovulation day. The moment that you get a positive pregnancy test, you’re about four weeks pregnant.
Often the dates may not match up with the first day of your last period, especially in a fresh transfer and the IVF cycle itself may change the length of your cycle or the time to ovulation.
Pregnancy dating with IVF and a frozen embryo transfer (FET)
In the case of a frozen embryo transfer (FET) you may go through a natural cycle or a medicated cycle. Let’s say you do a medicated cycle and your lining isn’t great. You may need to be on estrogen for longer. So it could be that you’re spending three weeks, for example, in the first part of the frozen embryo transfer cycle before you start your progesterone prior to your transfer. Based on this, you go by the day that the embryo was transferred. So, on the day of the embryo transfer you’re technically almost 3 weeks pregnant, provided that the embryo sticks and you get a positive pregnancy test.
Pregnancy dating: Day 5, Day 6 or Day 7 embryo – does this change the dating?
Does it matter if it’s a day-five embryo, a day-six embryo, or a day-seven embryo – does that change the dating of the pregnancy? In short, no. We assume that when we’re transferring a blastocyst (the specific stage of an embryo’s development) it is a day-five embryo.
When we’re thinking about dating a pregnancy after a frozen embryo transfer, we assume that they’re all universally day-five embryos.
It’s important to note that the pregnancy rates can vary between day-five, day-six and day-seven embryos and your doctor will discuss this with you when planning your transfer.
Pregnancy dating after a miscarriage
Sometimes you can become pregnant right after experiencing a miscarriage. The bleeding that happens with a miscarriage is not a real period so we can’t use that as the first day of the last period when we’re thinking about dating the pregnancy.
In this situation, you can’t use your menstrual cycle to date the pregnancy, and calculating your dates with an ultrasound is the most reliable source.
Pregnancy dating with ultrasounds
Generally, in most situations relying on ultrasound for dating a pregnancy is reliable and most accurate.
When we’re considering someone’s risk of a miscarriage, we are reassured when we see good interval growth on ultrasound. It is very reassuring knowing that if you do one ultrasound today and then repeat the ultrasound ten days later you want to see ten days worth of growth between those two ultrasounds – this shows that there is a good growing embryo and that the pregnancy is progressing as expected.
Pregnancy due date
A very important point I want to make is that once the due date is established, it does not change.
A well-established due date either based on your menstrual cycle and/or an ultrasound does not allow for any change. And usually, if your cycles are regular the two would agree.
It is very common for people to think a due date changes based on the size or growth of the baby. If the due date changed based on the fetus’s growth, then it would be impossible to measure the growth of the baby(!). For example, if at a 16 week ultrasound the baby is showing as being two weeks behind in size, the baby is still in fact 16 weeks (the baby does not get dated to 14 weeks).
The growth of baby is a good indicator of how well the baby is doing in utero and an OB may do ultrasounds in the third trimester to monitor the baby’s growth and well-being.