Ultrasound image of a tiny blob

a six-week embryo, about the size of a peaImage: Yiming Chen (Getty Images)

In the course of Supreme Court decision allow Texas plans to ban abortions after about six weeks of pregnancy, a few people – I don’t mention names – have shown that they have no idea how weeks are counted in pregnancy. Folks, the day you have sex to have a baby, you are considered pregnant as early as week two. And there is no way you can possibly know that you are pregnant until four or five weeks.

This is because the gestational age (weeks of pregnancy) does not depend on how long you have an embryo or fetus in your body, but rather how long has it been since the first day of your last period. Let’s go through this using a textbook example of a woman (let’s call her Jane) with a textbook-perfect cycle. She starts her period on January 1st and only has sex once in her life, resulting in pregnancy. These dates vary from person to person, so this is just an example:

  • January 1: Jane’s period begins. She loses the lining of the uterus. She is definitely not pregnant.
  • January 12th or so: She’s having sex.
  • January 14th: She is ovulating (an egg is released from her ovary). Shortly afterwards, the egg cell meets one of the sperm cells from a few days ago.
  • January 21st: The developing embryo nests in her uterus. She is now pregnant – more precisely in the third week of pregnancy.
  • Jan 28: Pregnancy hormones are high enough that maybe, just maybe, a very sensitive pregnancy test could detect them.
  • January 30th: Jane, looking at the calendar like a hawk, notices that her period is late. She is more than four weeks pregnant.

So you cannot find out that you are pregnant until the gestational age is “four weeks” at the earliest – which means that the embryo has only been implanted in your uterus for one week and only exists for two weeks. But the dates are counted as if the pregnancy started on January 1st.

This is an optimistic example for many reasons. For example, most people’s period doesn’t come on a textbook 28-day cycle. If your period tends to be anywhere between 29 and 34 days, you won’t be wondering until a little later. And this assumes that you are watching the calendar; If you are not expecting to become pregnant, you may not realize you are late for a week or two. The American Pregnancy Association notes that most women find that they are pregnant four to seven weeks Gestational age (that’s two to five weeks after the sex that leads to conception).

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Missing periods isn’t a great sign of pregnancy for everyone, either. Some people have irregular periods, especially if they are using some form of birth control that allows them to skip their periods or give them easier periods. In the meantime, after the embryo is implanted, there is a possibility of bleeding after three weeks and you might be thinking, “hm, my period is a little lighter and earlier than usual,” but don’t worry. There are also medical reasons a person might miss their period for reasons other than pregnancy.

So what about pregnancy symptoms? Morning sickness, for example, usually doesn’t appear until the fifth to seventh week, and some people don’t get morning sickness at all.

If you go to a doctor or clinic for a pregnancy test, we add more time. For example, suppose you think your period might be a week late and find your way to the clinic a few days later. You could easily be six weeks old before even getting a test.

This post was originally published in May 2019, in response to a similar law in Georgia. It was updated on September 3, 2021 in connection with the Supreme Court’s decision to uphold an abortion ban in Texas.